fix(worktree): preserve index-hidden changes

[testikun]

Problem

Closes #472. Automatic worktree reclamation trusted git status and non-force git worktree remove to detect local work. assume-unchanged and skip-worktree index flags can hide a modified tracked file, allowing the only copy to be deleted.

Value

A child worktree is preserved whenever its index contains state that can hide local changes. This prevents silent loss of uncommitted work while retaining the existing automatic cleanup behavior for provably clean worktrees.

Approach

• Read a bounded, NUL-delimited git ls-files -v -z inventory before cleanup.
• Preserve the worktree for assume-unchanged, skip-worktree, unknown entries, truncated output, or inspection failure.
• Keep the check read-only: no index flag clearing, reset, clean, checkout, or force removal.
• Add real temporary-Git-repository regression coverage for both flags, clean and modified files, index-byte preservation, unusual paths, and malformed/unavailable inventory.
• Record the design and ablation result in docs/design/WORKTREE_INDEX_SAFETY.md.

Validation

• node --test --experimental-strip-types tests/extensions/shared/worktree.test.ts passed: 37/37.
• bun run check passed: config contract, discipline ledger, Web build/typecheck, format, lint, and TypeScript typecheck.
• bun run test passed: 1476 passed, 1 platform-specific test skipped, 0 failed; Web suite 113/113.
• Ablation: removing the index inventory gate reproduced the reported unsafe cleanup for both hidden index flags, so the gate is required.
• Runtime Pi smoke: not applicable; this is a local Git cleanup invariant.

Impact

• User-visible behavior: worktrees with hidden or uncertain index state remain available for inspection instead of being automatically removed.
• Model-visible context/tools: none.
• Runtime/lifecycle: cleanup gains a fail-closed, read-only index inspection step.
• Persisted config/data: none.
• Compatibility/risk: ordinary clean worktrees and committed work retain existing behavior; Git inventory is bounded to 1 MiB and unusual paths are handled with NUL framing.