collab-stack: import collab stack with local repairs

Start from the pinned `origin/dev/friel/collab-stack` snapshot and fold in the
local follow-up repairs that made that imported stack usable here: watchdog
spawn/registration plumbing, deferred-tool fallback behavior, collab discovery
fixture alignment, schema mirroring, and subagent-panel fixes.

Original imported source:
- source ref: `refs/remotes/origin/dev/friel/collab-stack`
- source tip: `599ed9dc05eafd116192184bd54a2a55a2c49366`
- original base: `c1d18ceb6f22ae3acd67bbd6badad0f475b31dfc`

codex-rs/app-server-protocol/schema/json/v2/ThreadInputActivityResponse.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "ThreadInputActivityResponse",
+  "type": "object"
+}