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/templates/myext-template.md

@@ -0,0 +1,24 @@
+# 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

@@ -0,0 +1,18 @@
+# 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