Tasks: Notification Actions + Worktree Diff

• T001 [US1] notif.rs: mark-read action + action_event() pure fn

• unit tests; actor.rs UiEvent::MarkSeen; app.rs arm (pane.mark_seen + refresh).

• T002 [US2] git.rs git_diff + fixture unit test (tracked, untracked, binary, rename-as-is, non-repo None).
• T003 [US2] Proto WORKSPACE_DIFF (count 34→35) + router h_workspace_diff (handle-or-id, non-repo → BAD_PARAMS) + integration tests.
• T004 [US2] CLI workspace diff (human + --json) + cli.rs test.
• T005 Docs: docs/08-ipc.md row + docs/adr/0011-diff-and-deferrals.md.
• T006 Gates: fmt, clippy (zero new), full tests; commits.