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>

documentation/ARCH__FUTURE.md

@@ -46,7 +46,7 @@ Full API reference: [`docs/OPEN_WEBUI_API.md`](../docs/OPEN_WEBUI_API.md)
 
 ## 2. Orchestrator Tool Expansions
 
-**Status:** Ongoing. Current tool count: 44. Previously planned tools are all complete.
+**Status:** Ongoing. Current tool count: 45. Previously planned tools are all complete.
 
 ### Completed
 All originally planned tools are live: `cortex_restart`, `cortex_logs`, `http_fetch`,
@@ -58,10 +58,17 @@ All originally planned tools are live: `cortex_restart`, `cortex_logs`, `http_fe
 via `context_loader.py` (`--- System --- Current date and time: ...`). A dedicated
 `datetime_now` tool is not needed — the timestamp is always in context.
 
+### Completed Round 2
+| Tool | Notes |
+|---|---|
+| `session_search` | `tools/files.py` — full-text grep across session logs; params: `query`, `limit` (max 20); own sessions only via ContextVars. 2026-05-08 |
+| `reminders due dates` | `tools/reminders.py` — optional `due: YYYY-MM-DD` on `reminders_add`; `load_due_reminders()` suppresses future-dated entries from context. 2026-05-08 |
+| `spawn_agent` | `tools/agents.py` — sync sub-agent via role model; semaphore per host (`max_concurrent` in host schema); `asyncio.wait_for` timeout; admin-only. 2026-05-08 |
+
+### Remaining Round 2
+
 | Tool | Module | Priority | Description |
 |---|---|---|---|
-| `session_search` | new `search.py` or `files.py` | High | Full-text search across past session logs. The UI search already exists (`GET /sessions/search`) — this exposes it to the orchestrator so the agent can answer "what did we discuss about X last month?" |
-| `reminders due dates` | `reminders.py` | Medium | Add optional `due` field to `reminders_add`. Surface only due/overdue reminders in context rather than the full flat list. Makes reminders time-aware rather than always-on noise. |
 | `http_post` | `web.py` | Medium | POST to an external URL — for webhooks, REST APIs, form submissions. Requires a per-user host allowlist (same pattern as `email_send`) to prevent misuse. |
 | `nc_talk_history` | `notify.py` | Medium | Read recent messages from a Nextcloud Talk conversation. The bot can send but cannot read — adding read capability gives it full context before replying. |
 | `task_list` priority filter | `tasks.py` | Low | `task_list` accepts `status` but not `priority`. Add `priority` param so the agent can ask "what are my high-priority tasks?" without returning everything. |