fix(mcp): false worktree-mismatch warning when worktree has its own index (#926)

[Allicai]

Closes #926

What was wrong

Two stale-cache bugs in ToolHandler combined to produce a false "index belongs to a different git working tree" warning even when a linked worktree had its own .codegraph/ index and the tool was called with an explicit projectPath pointing at it.

Bug A — projectCache staleness: a prior tool call with projectPath=<wt> (before the worktree index existed) cached wt → mainRepoCG. After the worktree index was created the stale entry persisted, so getCodeGraph(wt) kept returning the main repo's CodeGraph.

Bug B — worktreeMismatchCache key collision: the mismatch cache was keyed on startPath alone. A call without projectPath (where startPath = defaultProjectHint = wt, indexRoot = mainRepo) cached a legitimate mismatch under key "wt". A later call with projectPath=wt (where indexRoot = wt) hit the same key and returned the stale entry.

Fix

• Fix A (getCodeGraph): when a cached entry's root is a parent of projectPath, re-run findNearestCodeGraphRoot; evict and fall through if a closer index now exists.
• Fix B (worktreeMismatchFor): key the mismatch cache on ${startPath}\0${indexRoot} so the two call shapes above get independent entries.

Tests

Added a regression describe block to __tests__/worktree-detection.test.ts covering the two fixed scenarios and the correct null result for (worktree, worktree).