docs: resolve remaining 3 design questions (spawn ownership, gate UX, mesh timeout)

- Spawn calls: runner owns all runtime_adapter.spawn() calls; tiers write
  status=pending briefs to blackboard, runner's spawn loop acts on them.
  Gate logic lives in the spawn loop — no gate plumbing needed in agents.

- Gate approval UX: Signal reply via Hans + direct CLI both supported.
  Both write gate_approved to blackboard; runner doesn't care which path.
  Hans uses pending_gates.json for multi-run disambiguation.

- T3 mesh timeout: escalate to T2 (domain boundary problem). If T2 also
  exhausts retry budget, normal escalation ladder handles it. No force-commit.

Add pending_gates.json to directory structure and buildspec.
Update runner step in build order with full spawn loop responsibilities.

docs/buildspec.md

@@ -74,6 +74,8 @@ agent-teams/
 ├── runs/                    — runtime state, one subdir per run_id
 │   └── .gitkeep
 │
+├── pending_gates.json       — live file: gates currently awaiting approval (written by runner, read by Hans)
+│
 └── README.md
 ```
 
@@ -488,7 +490,7 @@ T1 completes integration
 7. `core/escalation.py` — retry + failure routing logic (called by tiers, not runner centrally)
 8. `adapters/runtime/openclaw.py` — wire up sessions_spawn + personality injection
 9. `adapters/runtime/claude_code.py` — coding agent runtime, personality via --system-prompt
-10. `core/team_runner.py` — full run lifecycle: gate logic (gate_pending halt loop, gate_approved resume), path amendment monitor, T1 failure + terminal escalation only
+10. `core/team_runner.py` — full run lifecycle: spawn loop (monitors briefs table for `status=pending`, calls runtime_adapter.spawn()), gate logic (gate_pending halt, writes pending_gates.json, gate_approved/rejected resume), path amendment monitor, T3 mesh timeout → T2 escalation, T1 failure + terminal escalation only
 11. `cli/agency.py` — run, watch, inspect, approve, reject, pause, resume; `watch` tails blackboard events and renders live log; `inspect` renders run tree
 12. `prompts/` — fallback tier prompts (used when no agent_personality set)
 13. `adapters/vcs/github.py` — PR creation + branch management