Add modular extension system (#1551)

* Add modular extension system for Spec Kit

Implement a complete extension system that allows third-party developers
to extend Spec Kit functionality through plugins.

## Core Features
- Extension discovery and loading from local and global directories
- YAML-based extension manifest (extension.yml) with metadata and capabilities
- Command extensions: custom slash commands with markdown templates
- Hook system: pre/post hooks for generate, task, and sync operations
- Extension catalog for discovering and installing community extensions
- SPECKIT_CATALOG_URL environment variable for catalog URL override

## Installation Methods
- Catalog install: `specify extension add <name>`
- URL install: `specify extension add <name> --from <url>`
- Dev install: `specify extension add --dev <path>`

## Implementation
- ExtensionManager class for lifecycle management (load, enable, disable)
- Support for extension dependencies and version constraints
- Configuration layering (global → project → extension)
- Hook conditions for conditional execution

## Documentation
- RFC with design rationale and architecture decisions
- API reference for extension developers
- Development guide with examples
- User guide for installing and managing extensions
- Publishing guide for the extension catalog

## Included
- Extension template for bootstrapping new extensions
- Comprehensive test suite
- Example catalog.json structure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Update Jira extension to v2.1.0 in catalog

Adds 2-level mode support (Epic → Stories only).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Address PR review feedback

- Fix Zip Slip vulnerability in ZIP extraction with path validation
- Fix keep_config option to actually preserve config files on removal
- Add URL validation for SPECKIT_CATALOG_URL (HTTPS required, localhost exception)
- Add security warning when installing from custom URLs (--from flag)
- Empty catalog.json so organizations can ship their own catalogs
- Fix markdown linter errors (MD040: add language to code blocks)
- Remove redundant import and fix unused variables in tests
- Add comment explaining empty except clause for backwards compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add comprehensive organization catalog customization docs

- Explain why default catalog is empty (org control)
- Document how to create and host custom catalogs
- Add catalog JSON schema reference
- Include use cases: private extensions, curated catalogs, air-gapped environments
- Add examples for combining catalog with direct installation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix test assertions for extension system data structures

- Update test_config_backup_on_remove to use new subdirectory structure
  (.backup/test-ext/file.yml instead of .backup/test-ext-file.yml)
- Update test_full_install_and_remove_workflow to handle registered_commands
  being a dict keyed by agent name instead of a flat list

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Address Copilot review feedback

- Fix localhost URL check to use parsed.hostname instead of netloc.startswith()
  This correctly handles URLs with ports like localhost:8080
- Fix YAML indentation error in config-template.yml (line 57)
- Fix double space typo in example.md (line 172)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add catalog.example.json as reference for organizations

The main catalog.json is intentionally empty so organizations can ship
their own curated catalogs. This example file shows the expected schema
and structure for creating organization-specific catalogs.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Address remaining Copilot security and logic review feedback

- Fix Zip Slip vulnerability by using relative_to() for safe path validation
- Add HTTPS validation for extension download URLs
- Backup both *-config.yml and *-config.local.yml files on remove
- Normalize boolean values to lowercase for hook condition comparisons
- Show non-default catalog warning only once per instance

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Ignoring linter for extensions directory

---------

Co-authored-by: iamaeroplane <michal.bachorik@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Manfred Riem <manfred.riem@microsoft.com>

extensions/template/README.md

@@ -0,0 +1,79 @@
+# Extension Template
+
+Starter template for creating a Spec Kit extension.
+
+## Quick Start
+
+1. **Copy this template**:
+
+   ```bash
+   cp -r extensions/template my-extension
+   cd my-extension
+   ```
+
+2. **Customize `extension.yml`**:
+   - Change extension ID, name, description
+   - Update author and repository
+   - Define your commands
+
+3. **Create commands**:
+   - Add command files in `commands/` directory
+   - Use Markdown format with YAML frontmatter
+
+4. **Create config template**:
+   - Define configuration options
+   - Document all settings
+
+5. **Write documentation**:
+   - Update README.md with usage instructions
+   - Add examples
+
+6. **Test locally**:
+
+   ```bash
+   cd /path/to/spec-kit-project
+   specify extension add --dev /path/to/my-extension
+   ```
+
+7. **Publish** (optional):
+   - Create GitHub repository
+   - Create release
+   - Submit to catalog (see EXTENSION-PUBLISHING-GUIDE.md)
+
+## Files in This Template
+
+- `extension.yml` - Extension manifest (CUSTOMIZE THIS)
+- `config-template.yml` - Configuration template (CUSTOMIZE THIS)
+- `commands/example.md` - Example command (REPLACE THIS)
+- `README.md` - Extension documentation (REPLACE THIS)
+- `LICENSE` - MIT License (REVIEW THIS)
+- `CHANGELOG.md` - Version history (UPDATE THIS)
+- `.gitignore` - Git ignore rules
+
+## Customization Checklist
+
+- [ ] Update `extension.yml` with your extension details
+- [ ] Change extension ID to your extension name
+- [ ] Update author information
+- [ ] Define your commands
+- [ ] Create command files in `commands/`
+- [ ] Update config template
+- [ ] Write README with usage instructions
+- [ ] Add examples
+- [ ] Update LICENSE if needed
+- [ ] Test extension locally
+- [ ] Create git repository
+- [ ] Create first release
+
+## Need Help?
+
+- **Development Guide**: See EXTENSION-DEVELOPMENT-GUIDE.md
+- **API Reference**: See EXTENSION-API-REFERENCE.md
+- **Publishing Guide**: See EXTENSION-PUBLISHING-GUIDE.md
+- **User Guide**: See EXTENSION-USER-GUIDE.md
+
+## Template Version
+
+- Version: 1.0.0
+- Last Updated: 2026-01-28
+- Compatible with Spec Kit: >=0.1.0