feat: implement git repository management

- Add backend API for git repository CRUD (create, list, delete)
- Support bare repository initialization and mirror cloning
- Add cascade delete for repositories when project is deleted
- Add frontend page for repository management per project
- Update project page with link to repositories
- Add repo_base_path to config
- Quality gates: ruff, mypy, typecheck, lint, build all pass

openspec/changes/git-repo-management/proposal.md

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+## Why
+
+Projects exist but users cannot create or manage git repositories. The platform needs to allow users to create bare repositories, clone external ones, and manage them within projects.
+
+## What Changes
+
+- Add backend API for git repository CRUD operations
+- Implement bare repository initialization on disk
+- Support cloning external repositories as mirrors
+- Add frontend page for repository management
+- Integrate with existing project structure
+
+## Capabilities
+
+### New Capabilities
+- `git-repo-management`: Full git repository lifecycle within projects
+
+### Modified Capabilities
+- `project-management`: Include repositories in project responses
+
+## Impact
+
+- New API endpoints under `/projects/{id}/repositories`
+- Disk storage at `/data/repos/{user_id}/{project_id}/{repo_name}.git`
+- Frontend repository list and creation UI