feat(templates): add pluggable template system with packs, catalog, resolver, and CLI commands

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>

templates/template/README.md

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+# My Template Pack
+
+A custom template pack for Spec Kit.
+
+## Overview
+
+This template pack provides customized artifact templates for your development workflow.
+
+## Templates Included
+
+| Template | Type | Description |
+|----------|------|-------------|
+| `spec-template` | artifact | Custom feature specification template |
+
+## Installation
+
+```bash
+# Install from local directory (during development)
+specify template add --dev /path/to/this/directory
+
+# Install from catalog (after publishing)
+specify template add my-template-pack
+```
+
+## Usage
+
+Once installed, templates are automatically resolved by the Spec Kit scripts.
+When you run `specify specify` or create a new feature, your custom templates
+will be used instead of the core defaults.
+
+## Template Types
+
+- **artifact** — Document scaffolds (spec.md, plan.md, tasks.md, etc.)
+- **command** — AI agent prompts (the files in `.claude/commands/`, etc.)
+- **script** — Custom scripts that replace core scripts
+
+## Development
+
+1. Edit templates in the `templates/` directory
+2. Test with: `specify template add --dev .`
+3. Verify with: `specify template resolve spec-template`
+
+## Publishing
+
+See the [Template Publishing Guide](../../docs/TEMPLATE-PUBLISHING-GUIDE.md) for details.
+
+## License
+
+MIT

templates/template/template-pack.yml

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+schema_version: "1.0"
+
+template_pack:
+  # CUSTOMIZE: Change 'my-template-pack' to your template pack ID (lowercase, hyphen-separated)
+  id: "my-template-pack"
+
+  # CUSTOMIZE: Human-readable name for your template pack
+  name: "My Template Pack"
+
+  # 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 template pack 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-templates-my-pack"
+
+  # REVIEW: License (MIT is recommended for open source)
+  license: "MIT"
+
+# Requirements for this template pack
+requires:
+  # CUSTOMIZE: Minimum spec-kit version required
+  speckit_version: ">=0.1.0"
+
+# Templates provided by this pack
+provides:
+  templates:
+    # CUSTOMIZE: Define your artifact templates
+    # Artifact templates are document scaffolds (spec.md, plan.md, etc.)
+    - type: "artifact"
+      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: "artifact"
+    #   name: "plan-template"
+    #   file: "templates/plan-template.md"
+    #   description: "Custom plan template"
+    #   replaces: "plan-template"
+
+    # Command templates (AI agent prompts)
+    # - type: "command"
+    #   name: "specify"
+    #   file: "commands/specify.md"
+    #   description: "Custom specification command"
+    #   replaces: "specify"
+
+    # Script templates
+    # - 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"
+  - "template"

templates/template/templates/spec-template.md

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+# Feature Specification
+
+> Replace this with your actual specification content.
+
+## Overview
+
+Brief description of the feature.
+
+## Requirements
+
+- Requirement 1
+- Requirement 2
+
+## Design
+
+Describe the design approach.
+
+## Acceptance Criteria
+
+- [ ] Criterion 1
+- [ ] Criterion 2