feat: spawn_agent tool + host max_concurrent + docs

Adds a synchronous sub-agent spawning tool that lets the orchestrator
delegate tasks to a specific role's model and tool set.

- cortex/tools/agents.py: spawn_agent(task, role, tier, timeout, max_rounds)
  - Supports local_openai and gemini_api model types
  - Per-host asyncio semaphore (keyed by host_id or model type)
  - asyncio.wait_for() enforces timeout; admin-only tool
- cortex/model_registry.py: max_concurrent field in host schema (default 3,
  clamped 1-20); backfilled on _normalize() for existing hosts
- cortex/routers/local_llm.py + local_llm.html: "Max parallel" number input
  in host add/edit forms
- cortex/tools/__init__.py: spawn_agent registered in TOOL_CATEGORIES["Agents"],
  _CALLABLES, TOOL_ROLES (admin), and _ALL_DECLARATIONS
- Docs: TOOLS.md count 44→45, spawn_agent section; HELP.md tool table updated;
  ARCH__FUTURE.md Round 2 completed items; TODO__Agents.md spawn_agent checked;
  CLAUDE.md tool count and list updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/TOOLS.md

@@ -1,6 +1,6 @@
 # Tool Reference
 
-> This reference covers all 44 orchestrator tools available when the ⚡ toggle is on.
+> This reference covers all 45 orchestrator tools available when the ⚡ toggle is on.
 > Tools are invoked automatically by the orchestrator — you don't call them directly.
 
 ¹ **Admin only** — requires the `admin` role. Invisible to regular users.  
@@ -113,3 +113,11 @@ Private, durable notes visible only to the orchestrator — not surfaced to user
 | `agent_notes_write` | Overwrite the notes file completely |
 | `agent_notes_append` | Append a timestamped entry (keeps last 3 backups automatically) |
 | `agent_notes_clear` | Erase all notes (backs up first) |
+
+## Agents ¹
+
+Spawn sub-agents that run their own tool loop using a specific role's model and tools.
+
+| Tool | What it does |
+|---|---|
+| `spawn_agent` ¹ | Spawn a sub-agent synchronously — blocks until the task completes or times out. Params: `task`, `role` (default `chat`), `tier` (1–4, default 1), `timeout` seconds, `max_rounds` override. Only works with `local_openai` and `gemini_api` models. |