feat(presets): pluggable preset system with template/command overrides, catalog, and resolver

- Rename 'template packs' to 'presets' to avoid naming collision with core templates
- PresetManifest, PresetRegistry, PresetManager, PresetCatalog, PresetResolver in presets.py
- Extract CommandRegistrar to agents.py as shared infrastructure
- CLI: specify preset list/add/remove/search/resolve/info
- CLI: specify preset catalog list/add/remove
- --preset option on specify init
- Priority-based preset stacking (--priority, lower = higher precedence)
- Command overrides registered into all detected agent directories (17+ agents)
- Extension command safety: skip registration if target extension not installed
- Multi-catalog support: env var, project config, user config, built-in defaults
- resolve_template() / Resolve-Template in bash/PowerShell scripts
- Self-test preset: overrides all 6 core templates + 1 command
- Scaffold with 4 examples: core/extension template and command overrides
- Preset catalog (catalog.json, catalog.community.json)
- Documentation: README.md, ARCHITECTURE.md, PUBLISHING.md
- 110 preset tests, 253 total tests passing

presets/scaffold/README.md

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+# My Preset
+
+A custom preset for Spec Kit. Copy this directory and customize it to create your own.
+
+## Templates Included
+
+| Template | Type | Description |
+|----------|------|-------------|
+| `spec-template` | template | Custom feature specification template (overrides core and extensions) |
+| `myext-template` | template | Override of the myext extension's report template |
+| `speckit.specify` | command | Custom specification command (overrides core) |
+| `speckit.myext.myextcmd` | command | Override of the myext extension's myextcmd command |
+
+## Development
+
+1. Copy this directory: `cp -r presets/scaffold my-preset`
+2. Edit `preset.yml` — set your preset's ID, name, description, and templates
+3. Add or modify templates in `templates/`
+4. Test locally: `specify preset add --dev ./my-preset`
+5. Verify resolution: `specify preset resolve spec-template`
+6. Remove when done testing: `specify preset remove my-preset`
+
+## Manifest Reference (`preset.yml`)
+
+Required fields:
+- `schema_version` — always `"1.0"`
+- `preset.id` — lowercase alphanumeric with hyphens
+- `preset.name` — human-readable name
+- `preset.version` — semantic version (e.g. `1.0.0`)
+- `preset.description` — brief description
+- `requires.speckit_version` — version constraint (e.g. `>=0.1.0`)
+- `provides.templates` — list of templates with `type`, `name`, and `file`
+
+## Template Types
+
+- **template** — Document scaffolds (spec-template.md, plan-template.md, tasks-template.md, etc.)
+- **command** — AI agent workflow prompts (e.g. speckit.specify, speckit.plan)
+- **script** — Custom scripts (reserved for future use)
+
+## Publishing
+
+See the [Preset Publishing Guide](../PUBLISHING.md) for details on submitting to the catalog.
+
+## License
+
+MIT

presets/scaffold/commands/speckit.myext.myextcmd.md

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+---
+description: "Override of the myext extension's myextcmd command"
+---
+
+<!-- Preset override for speckit.myext.myextcmd -->
+
+You are following a customized version of the myext extension's myextcmd command.
+
+When executing this command:
+
+1. Read the user's input from $ARGUMENTS
+2. Follow the standard myextcmd workflow
+3. Additionally, apply the following customizations from this preset:
+   - Add compliance checks before proceeding
+   - Include audit trail entries in the output
+
+> CUSTOMIZE: Replace the instructions above with your own.
+> This file overrides the command that the "myext" extension provides.
+> When this preset is installed, all agents (Claude, Gemini, Copilot, etc.)
+> will use this version instead of the extension's original.

presets/scaffold/commands/speckit.specify.md

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+---
+description: "Create a feature specification (preset override)"
+scripts:
+  sh: scripts/bash/create-new-feature.sh "{ARGS}"
+  ps: scripts/powershell/create-new-feature.ps1 "{ARGS}"
+---
+
+## User Input
+
+```text
+$ARGUMENTS
+```
+
+Given the feature description above:
+
+1. **Create the feature branch** by running the script:
+   - Bash: `{SCRIPT} --json --short-name "<short-name>" "<description>"`
+   - The JSON output contains BRANCH_NAME and SPEC_FILE paths.
+
+2. **Read the spec-template** to see the sections you need to fill.
+
+3. **Write the specification** to SPEC_FILE, replacing the placeholders in each section
+   (Overview, Requirements, Acceptance Criteria) with details from the user's description.

presets/scaffold/preset.yml

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+schema_version: "1.0"
+
+preset:
+  # CUSTOMIZE: Change 'my-preset' to your preset ID (lowercase, hyphen-separated)
+  id: "my-preset"
+
+  # CUSTOMIZE: Human-readable name for your preset
+  name: "My Preset"
+
+  # CUSTOMIZE: Update version when releasing (semantic versioning: X.Y.Z)
+  version: "1.0.0"
+
+  # CUSTOMIZE: Brief description (under 200 characters)
+  description: "Brief description of what your preset provides"
+
+  # CUSTOMIZE: Your name or organization name
+  author: "Your Name"
+
+  # CUSTOMIZE: GitHub repository URL (create before publishing)
+  repository: "https://github.com/your-org/spec-kit-preset-my-preset"
+
+  # REVIEW: License (MIT is recommended for open source)
+  license: "MIT"
+
+# Requirements for this preset
+requires:
+  # CUSTOMIZE: Minimum spec-kit version required
+  speckit_version: ">=0.1.0"
+
+# Templates provided by this preset
+provides:
+  templates:
+    # CUSTOMIZE: Define your template overrides
+    # Templates are document scaffolds (spec-template.md, plan-template.md, etc.)
+    - type: "template"
+      name: "spec-template"
+      file: "templates/spec-template.md"
+      description: "Custom feature specification template"
+      replaces: "spec-template"  # Which core template this overrides (optional)
+
+    # ADD MORE TEMPLATES: Copy this block for each template
+    # - type: "template"
+    #   name: "plan-template"
+    #   file: "templates/plan-template.md"
+    #   description: "Custom plan template"
+    #   replaces: "plan-template"
+
+    # OVERRIDE EXTENSION TEMPLATES:
+    # Presets sit above extensions in the resolution stack, so you can
+    # override templates provided by any installed extension.
+    # For example, if the "myext" extension provides a spec-template,
+    # the preset's version above will take priority automatically.
+
+    # Override a template provided by the "myext" extension:
+    - type: "template"
+      name: "myext-template"
+      file: "templates/myext-template.md"
+      description: "Override myext's report template"
+      replaces: "myext-template"
+
+    # Command overrides (AI agent workflow prompts)
+    # Presets can override both core and extension commands.
+    # Commands are automatically registered into all detected agent
+    # directories (.claude/commands/, .gemini/commands/, etc.)
+
+    # Override a core command:
+    - type: "command"
+      name: "speckit.specify"
+      file: "commands/speckit.specify.md"
+      description: "Custom specification command"
+      replaces: "speckit.specify"
+
+    # Override an extension command (e.g. from the "myext" extension):
+    - type: "command"
+      name: "speckit.myext.myextcmd"
+      file: "commands/speckit.myext.myextcmd.md"
+      description: "Override myext's myextcmd command with custom workflow"
+      replaces: "speckit.myext.myextcmd"
+
+    # Script templates (reserved for future use)
+    # - type: "script"
+    #   name: "create-new-feature"
+    #   file: "scripts/bash/create-new-feature.sh"
+    #   description: "Custom feature creation script"
+    #   replaces: "create-new-feature"
+
+# CUSTOMIZE: Add relevant tags (2-5 recommended)
+# Used for discovery in catalog
+tags:
+  - "example"
+  - "preset"

presets/scaffold/templates/myext-template.md

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+# MyExt Report
+
+> This template overrides the one provided by the "myext" extension.
+> Customize it to match your needs.
+
+## Summary
+
+Brief summary of the report.
+
+## Details
+
+- Detail 1
+- Detail 2
+
+## Actions
+
+- [ ] Action 1
+- [ ] Action 2
+
+<!--
+  CUSTOMIZE: This template takes priority over the myext extension's
+  version of myext-template. The extension's original is still available
+  if you remove this preset.
+-->

presets/scaffold/templates/spec-template.md

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+# Feature Specification: [FEATURE NAME]
+
+**Created**: [DATE]
+**Status**: Draft
+
+## Overview
+
+[Brief description of the feature]
+
+## Requirements
+
+- [ ] Requirement 1
+- [ ] Requirement 2
+
+## Acceptance Criteria
+
+- [ ] Criterion 1
+- [ ] Criterion 2