feat: git diff panel (Files/Git tab in the file browser)

Adds a Git tab to the right-side file panel that shows the project
repository's diff and lets the user stage, unstage, commit, and discard
whole files in-session. Two comparison modes (Uncommitted vs HEAD, and the
branch vs its base — upstream tracking branch else default branch), auto-
selected by repo state on first open and pinned after explicit choice;
per-file expand/collapse with lazy syntax-highlighted diffs, +/- stats, and
binary/large-file placeholders. All git read and write logic lives in
apps/server via a new git_diff service: argv-safe execFile only (never a
shell), per-file paths validated repo-relative through pathGuard with a
realpath symlink-escape check, server-derived commit identity (the request
carries no author fields), and the write endpoints are deliberately absent
from the assistant tool registry. Reads are bounded (30s deadline, 10MB);
an index lock or an in-progress merge/rebase/cherry-pick/bisect surfaces as
"repository busy" and disables writes. The panel stays current via a client
git_diff_refresh session event (no new wire contract) coalesced across tab
open, mutations, turn completion, and pending-change apply. Discard is an
irrecoverable hard-delete behind a plain confirm that distinguishes
reverting a tracked file from deleting an untracked one.

apps/web/src/hooks/useSidebar.ts

@@ -186,6 +186,8 @@ function applyEvent(prev: SidebarResponse, event: import('./sessionEvents').Sess
     case 'chat_deleted':
     case 'chat_status':
     case 'refetch_messages':
+    case 'git_diff_refresh':
+      // Consumed by useGitDiff; no sidebar state change needed.
       return prev;
     case 'project_archived': {
       const next = prev.projects.filter((p) => p.id !== event.project_id);