fix(git): find .beads from nested worktrees (GH#509)

When worktrees are nested under the main repo (e.g.,
/project/.worktrees/feature/), bd now correctly finds .beads/ in the
parent repo.

The fix simplifies GetMainRepoRoot() to use `git rev-parse --git-common-dir`
which always returns the main repo's .git directory, regardless of whether
we're in a regular repo, a worktree, or a nested worktree.

- Simplified GetMainRepoRoot() implementation
- Added tests for nested worktree scenarios
- Updated CHANGELOG.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CHANGELOG.md

@@ -30,6 +30,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Worktree lifecycle management with sparse checkout for sync branches
   - Automatic detection and user-friendly warnings for worktree conflicts
 
+### Fixed
+
+- **`bd` now finds `.beads` from nested worktrees** (GH#509)
+  - When worktrees are nested under the main repo (e.g., `/project/.worktrees/feature/`),
+    `bd` now correctly finds `.beads/` in the parent repo
+  - Uses `git rev-parse --git-common-dir` to reliably locate the main repository root
+  - Works from any subdirectory within the nested worktree
+
 ## [0.29.0] - 2025-12-03
 
 ### Added