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/codex-api/tests/models_integration.rs

@@ -53,6 +53,7 @@ async fn models_client_hits_models_endpoint() {
     let response = ModelsResponse {
         models: vec![ModelInfo {
             slug: "gpt-test".to_string(),
+            request_model: None,
             display_name: "gpt-test".to_string(),
             description: Some("desc".to_string()),
             default_reasoning_level: Some(ReasoningEffort::Medium),