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.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/sessionEvents.ts

@@ -178,6 +178,12 @@ export interface RefetchMessagesEvent {
   type: 'refetch_messages';
 }
 
+// git-diff-panel Phase 1: emitted client-side to trigger a panel refresh.
+// Not a WS frame — no @boocode/contracts change required.
+export interface GitDiffRefreshEvent {
+  type: 'git_diff_refresh';
+}
+
 export type SessionEvent =
   | SessionRenamedEvent
   | ProjectCreatedEvent
@@ -204,7 +210,8 @@ export type SessionEvent =
   | ProjectUnarchivedEvent
   | ProjectUpdatedEvent
   | ChatStatusEvent
-  | RefetchMessagesEvent;
+  | RefetchMessagesEvent
+  | GitDiffRefreshEvent;
 type Listener = (event: SessionEvent) => void;
 
 const listeners = new Set<Listener>();