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>

CLAUDE.md

@@ -255,14 +255,14 @@ Cortex is running and stable. All channels are live:
 
 Active users: scott (inara), holly (tina), brian (wintermute)
 
-**40 orchestrator tools:** web_search, http_fetch,
-file_read/list/write, shell_exec, claude_allow_dir,
+**45 orchestrator tools:** web_search, http_fetch,
+file_read/list/write/session_search, shell_exec, claude_allow_dir,
 cortex_restart/logs/status/update,
 task_list/create/update/complete, cron_list/add/remove/toggle,
 reminders_add/list/remove/clear, scratch_read/write/append/clear,
 web_push, email_send, nc_talk_send,
 ae_journal_list/search/entries_list/entry_read/entry_create/entry_update/entry_disable/entry_append/entry_prepend,
-ae_task_list.
+ae_task_list, agent_notes_read/write/append/clear, spawn_agent.
 
 See `documentation/TODO__Agents.md` for the active task list.
 See `documentation/ROADMAP.md` for phases and what's next.