feat(git): add worktree branch roots

[pcristin]

Summary

Fixes #351.

Adds first-class git worktree awareness by introducing a git context resolver and using branch nodes as the graph root beneath each project:

• Detect git repos, linked worktrees, detached HEADs, branch slugs, canonical common root, HEAD/base SHAs, and git dirs.
• Add Project -[:HAS_BRANCH]-> Branch and reparent top-level structural containment to the branch node while preserving existing file/folder/function qualified names.
• Surface git/worktree metadata in list_projects and index_status.
• Add focused tests for non-git paths, linked worktrees, branch-root graph shape, and MCP status metadata.

Notes

• Pure C implementation, no new dependencies.
• New cbm_popen usage is limited to git metadata lookup and is documented in scripts/security-allowlist.txt.
• This PR is over the preferred 500-line guideline because it includes the resolver plus regression tests in one issue-scoped change.

Verification

• make -f Makefile.cbm build/c/test-runner passes.
• git diff --check passes.
• Touched files pass clang-format dry-run.
• scripts/test.sh was run; it fails in existing incremental dump/status tests with final summary 3513 passed, 112 failed, 16 skipped.
• scripts/lint.sh was run; it fails because cppcheck is not installed locally and because of existing broad clang-tidy findings outside this diff. The diff-specific formatting finding was fixed.