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

@@ -0,0 +1,46 @@
+# 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