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/tools/__init__.py

@@ -70,6 +70,7 @@ from tools.agent_notes import (
     agent_notes_append as _agent_notes_append,
     agent_notes_clear  as _agent_notes_clear,
 )
+from tools.agents import spawn_agent as _spawn_agent
 
 # ── Declaration imports ───────────────────────────────────────────────────────
 
@@ -84,6 +85,7 @@ import tools.reminders    as _mod_reminders
 import tools.scratch      as _mod_scratch
 import tools.notify       as _mod_notify
 import tools.agent_notes  as _mod_agent_notes
+import tools.agents       as _mod_agents
 
 # ── Tool categories — used by the Model Registry UI for grouped checkboxes ───
 
@@ -106,6 +108,7 @@ TOOL_CATEGORIES: dict[str, list[str]] = {
     ],
     "Aether Tasks":     ["ae_task_list"],
     "Agent Notes":      ["agent_notes_read", "agent_notes_write", "agent_notes_append", "agent_notes_clear"],
+    "Agents":           ["spawn_agent"],
 }
 
 # ── Callable registry ─────────────────────────────────────────────────────────
@@ -156,6 +159,7 @@ _CALLABLES: dict[str, callable] = {
     "agent_notes_write":         _agent_notes_write,
     "agent_notes_append":        _agent_notes_append,
     "agent_notes_clear":         _agent_notes_clear,
+    "spawn_agent":               _spawn_agent,
 }
 
 # ── Role-based access control ─────────────────────────────────────────────────
@@ -172,6 +176,7 @@ TOOL_ROLES: dict[str, str] = {
     "file_list":        "admin",
     "file_write":       "admin",
     "ae_task_list":     "admin",
+    "spawn_agent":      "admin",
     "email_send":       "admin",
     "nc_talk_send":     "admin",
 }
@@ -208,6 +213,7 @@ _ALL_DECLARATIONS: list[types.FunctionDeclaration] = (
     + _mod_ae_knowledge.DECLARATIONS
     + _mod_ae_tasks.DECLARATIONS
     + _mod_agent_notes.DECLARATIONS
+    + _mod_agents.DECLARATIONS
 )
 
 # Full Gemini Tool object (all tools — use get_tools_for_role() in production)