agent parallelism: background tools, bg spawn_agent, parallel tool batches, message queue ...

- backgroundable tools (terminal/ssh_exec/peer/spawn_agent/code_exec) detach
  via TaskManager with per-session/global/spawn caps, rate limit and TTL
- completion delivery both ways: task_update event (out-of-band) + pending
  notes injected into the next turn; tasks tool (list/check/wait/cancel)
- parallel tool-call batches (profile-level gate, off by default) with
  ToolStarted up-front, tagged event mux, call-order results, one save,
  plus dangling tool_call repair on session load
- user message queue instead of busy error: message_queued frame,
  back-to-back drain on the same socket, headless fallback on disconnect
- pin mcp<2 (v2 renames FastMCP with breaking API changes)
- docs: tasks.md (new), websocket/api/agent/config updates, tasks manual,
  spawn_agent background param, persona contract section

port: 8000/8001 -> 8099/8098 everywhere ...

navi runs on shared servers where 8000/8001 are usually taken. New
defaults: API 8099, navi_ui MCP 8098. Touched: config defaults
(navi_port, navi_ui_mcp_port, public_url, gnauth_redirect_uri),
navi-server launcher docs, env.template/.env.example, install.sh
health-check fallback, terminal client base_url, webclient dev configs
(useWebSocket, contentLinks, vite proxy), android url hint, docs.
Also made the navi_ui FastMCP constructor port settings-driven instead
of a hardcoded 8001 (it was overridden at start anyway).

server: NAVI_WEBCLIENT_ENABLED gate + navi-server launcher ...

NAVI_WEBCLIENT_ENABLED=false removes every web-facing route (/, /assets,
/images, /content-viewers, /content, /admin panel, /debug*) and skips
the navi_ui MCP server — its only consumer is the webclient. The REST
API and WS the terminal client uses stay fully intact, and so does the
/admin/* JSON API (still gated by require_admin). Static mounts use
check_dir=False so a tree without webclient/dist imports fine.

New navi-server console script (navi/server.py): uvicorn launcher that
reads NAVI_HOST/NAVI_PORT from settings — the systemd unit needs no
hardcoded values.

Default stays true (full web UI), so nothing changes for dev setups;
subprocess route tests cover both modes.