feat(p7): moteur git détaillé, API fichiers et watcher FS temps réel

Fondations de la refonte « vrai IDE IA worktree » (chantier P7→P12).

- git.ts : listChanges (status --porcelain=v2 -z + --numstat fusionnés),
  fileDiff (diff unifié borné, refus binaire, untracked via --no-index),
  stage/unstage/restore/clean, commitStaged, amendCommit (refus si poussé),
  fetchRemote, pull (ff-only|rebase), lastCommit, resolveGitDir,
  isSafeRelativePath ; worktreeStatus enrichi (compteurs staged/unstaged/
  conflict + dernier commit), champs additifs WorktreeGitStatus.
- core/fs-watcher.ts : FsWatcherService (chokidar, refcount + pool LRU borné,
  ignore .git sauf HEAD/index, débounce) → invalide factsCache, rediffuse
  worktree_update et émet worktree_changes.
- WorktreeManager : getWorktreeChanges/getFileDiff/stage/unstage/discard/
  fetch/pull/watch/unwatch/assertPathInWorktree ; commitWorktree { mode, amend }.
- routes/git.ts (changes/diff/stage/unstage/discard/fetch/pull) et
  routes/files.ts (GET/PUT contenu fichier, bornage strict au worktree) ;
  fs/list?includeFiles=1 (flag isFile).
- protocole ADDITIF (PROTOCOL_VERSION inchangé) : messages WS watch/unwatch
  (validés dans parseClientMessage) + worktree_changes poussé ciblé par la
  gateway ; front wsClient.watchWorktree + lib/git-api.ts + api.put.
- tests git/fs-watcher/protocole + acceptance-p7.mjs (ALL GREEN).
- dépendance : chokidar (serveur).

packages/shared/test/protocol.test.ts

@@ -128,6 +128,11 @@ describe('parseClientMessage — cas valides', () => {
     expect(parseClientMessage('{"type":"sub","topics":[]}')).toEqual({ type: 'sub', topics: [] });
   });
 
+  it('watch / unwatch (P7) avec repoId + path', () => {
+    expect(parseClientMessage('{"type":"watch","repoId":"r1","path":"/home/u/wt"}')).toEqual({ type: 'watch', repoId: 'r1', path: '/home/u/wt' });
+    expect(parseClientMessage('{"type":"unwatch","repoId":"r1","path":"/home/u/wt"}')).toEqual({ type: 'unwatch', repoId: 'r1', path: '/home/u/wt' });
+  });
+
   it('ping (champs superflus ignorés)', () => {
     expect(parseClientMessage('{"type":"ping","extra":42}')).toEqual({ type: 'ping' });
   });
@@ -152,6 +157,13 @@ describe('parseClientMessage — cas malformés', () => {
     expect(parseClientMessage('{"type":"hello","protocol":"1"}')).toBeNull();
   });
 
+  it('watch / unwatch : repoId ou path manquant / mal typé', () => {
+    expect(parseClientMessage('{"type":"watch","path":"/x"}')).toBeNull();
+    expect(parseClientMessage('{"type":"watch","repoId":"r1"}')).toBeNull();
+    expect(parseClientMessage('{"type":"watch","repoId":"","path":"/x"}')).toBeNull();
+    expect(parseClientMessage('{"type":"unwatch","repoId":"r1","path":42}')).toBeNull();
+  });
+
   it('attach : dimensions hors bornes ou non entières', () => {
     const make = (over: Record<string, unknown>): string =>
       JSON.stringify({ type: 'attach', sessionId: 's1', mode: 'interactive', cols: 80, rows: 24, ...over });