fix(pi): loosen plan-mode write gate to directory scope

[backnotprop]

Summary

• Plan-mode write/edit gate did an exact-path comparison against planFilePath, so /plannotator plans/ blocked every file inside plans/ and the agent fell back to bash cat > file to bypass it.
• Gate is now directory-scoped when planFilePath ends with a separator, has no file extension, or points to a subdirectory. Default PLAN.md behavior is unchanged.
• Extracted the logic to a pure function (isPlanWritePathAllowed) in tool-scope.ts with 6 new unit tests.

Repro that now works

$ /plannotator plans/
$ "make a simple plan for snake game"
→ agent writes plans/snake_game_plan.md (allowed), no bash fallback

Test plan

• bun test apps/pi-extension/tool-scope.test.ts — 9 pass, 0 fail
• tsc --noEmit -p apps/pi-extension/tsconfig.json — clean
• Manual: /plannotator plans/ in a fresh project, confirm plan writes succeed
• Manual: /plannotator PLAN.md (default), confirm behavior unchanged and writes to src/ still blocked
• Manual: /plannotator plans/auth.md, confirm sibling writes (plans/notes.md) allowed, writes outside blocked

[backnotprop]

Superseded by #595, which redesigns plan mode around agent-supplied file paths instead of the configured-path gate. That approach avoids both the sibling-unlock regression this PR introduced and the pre-existing submit-on-directory failure.