finish T-87: cold-start reap, kill-all-sessions, helper tests

Three remaining acceptance criteria for T-87:

1. Cold-start reap. The Claude extension's activate() now kills
   every leftover secondary tmux session for the current repo
   before any new spawn. activate runs before any UI mounts, so
   _nextSecondary's starting value of 1 is correct even when a
   previous run died abruptly (kill -9, OOM, force-quit). The
   deactivate() hook also calls reapSecondaries as a courtesy on
   explicit extension teardown — but Flutter's deactivate doesn't
   fire on app quit, so activate is the load-bearing path.

2. claude.kill-all-sessions actually kills server-side. The
   command previously called pane.close on every claude pane,
   which only kills the tmux client. It now also calls
   tmux.killAllForRepo to kill the sessions on the clide socket.

3. Tests. test/builtin/claude/tmux_session_test.dart covers
   killSession, listClideSessions, reapSecondaries, and
   killAllForRepo via the TmuxRunner override — no real shell-out
   in tests.

Co-Authored-By: Claude <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-06T14:41:33Z",
+  "exported_at": "2026-05-06T14:48:06Z",
   "decisions": [
     {
       "id": "D-1",