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/useSessionStream.ts

@@ -273,6 +273,10 @@ export function useSessionStream(sessionId: string | undefined) {
             return;
           }
           setState((s) => applyFrame(s, frame));
+          // Trigger git diff refresh after each completed assistant turn.
+          if (frame.type === 'message_complete') {
+            sessionEvents.emit({ type: 'git_diff_refresh' });
+          }
         } catch (err) {
           console.warn('bad ws frame', err);
         }