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/local_llm.html

@@ -403,6 +403,10 @@
                   <option value="openai">OpenAI-compatible (OpenRouter, etc.)</option>
                 </select>
               </div>
+              <div class="field" style="flex:0 0 auto; width:6rem">
+                <label>Max parallel</label>
+                <input type="number" name="max_concurrent" min="1" max="20" value="3" style="width:100%">
+              </div>
             </div>
             <div class="btn-row">
               <button type="submit" class="btn btn-primary btn-sm">Add Host</button>