Add Brownfield Bootstrap extension to community catalog (#2145)

- 4 commands: scan, bootstrap, validate, migrate for existing codebases
- 1 hook: after_init for auto-scanning project after spec-kit initialization
- Addresses community request in issue #1436 (30+ reactions)

README.md

@@ -186,6 +186,7 @@ The following community-contributed extensions are available in [`catalog.commun
 | Archive Extension | Archive merged features into main project memory. | `docs` | Read+Write | [spec-kit-archive](https://github.com/stn1slv/spec-kit-archive) |
 | Azure DevOps Integration | Sync user stories and tasks to Azure DevOps work items using OAuth authentication | `integration` | Read+Write | [spec-kit-azure-devops](https://github.com/pragya247/spec-kit-azure-devops) |
 | Branch Convention | Configurable branch and folder naming conventions for /specify with presets and custom patterns | `process` | Read+Write | [spec-kit-branch-convention](https://github.com/Quratulain-bilal/spec-kit-branch-convention) |
+| Brownfield Bootstrap | Bootstrap spec-kit for existing codebases — auto-discover architecture and adopt SDD incrementally | `process` | Read+Write | [spec-kit-brownfield](https://github.com/Quratulain-bilal/spec-kit-brownfield) |
 | Bugfix Workflow | Structured bugfix workflow — capture bugs, trace to spec artifacts, and patch specs surgically | `process` | Read+Write | [spec-kit-bugfix](https://github.com/Quratulain-bilal/spec-kit-bugfix) |
 | Canon | Adds canon-driven (baseline-driven) workflows: spec-first, code-first, spec-drift. Requires Canon Core preset installation. | `process` | Read+Write | [spec-kit-canon](https://github.com/maximiliamus/spec-kit-canon/tree/master/extension) |
 | CI Guard | Spec compliance gates for CI/CD — verify specs exist, check drift, and block merges on gaps | `process` | Read-only | [spec-kit-ci-guard](https://github.com/Quratulain-bilal/spec-kit-ci-guard) |