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/routers/local_llm.py

@@ -114,6 +114,11 @@ def _render(username: str, success: str = "", error: str = "") -> str:
                   <option value="openai"{ai}>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="{h.get('max_concurrent', 3)}" style="width:100%">
+              </div>
             </div>
             <div class="btn-row">
               <button type="submit" class="btn btn-secondary btn-sm">Save</button>
@@ -421,19 +426,20 @@ async def remove_google_account(request: Request, account_id: str):
 
 @router.post("/settings/local/host", include_in_schema=False)
 async def save_host(
-    request:   Request,
-    host_id:   str = Form(""),
-    label:     str = Form(""),
-    api_url:   str = Form(""),
-    api_key:   str = Form(""),
-    host_type: str = Form("openwebui"),
+    request:        Request,
+    host_id:        str = Form(""),
+    label:          str = Form(""),
+    api_url:        str = Form(""),
+    api_key:        str = Form(""),
+    host_type:      str = Form("openwebui"),
+    max_concurrent: int = Form(3),
 ):
     username = _get_user(request)
     if not username:
         return RedirectResponse("/login", status_code=302)
     if not api_url.strip():
         return HTMLResponse(_render(username, error="API URL is required."))
-    reg.save_host(username, host_id or None, label, api_url, api_key, host_type)
+    reg.save_host(username, host_id or None, label, api_url, api_key, host_type, max_concurrent)
     return HTMLResponse(_render(username, success="Host saved."))