context: task-note messages survive build()'s system filter ...

E2E caught a real delivery bug: the completion note was drained and
persisted (agent.task_notes_drained logged) but build() filtered out all
system-role history from session.context, so the LLM never saw it — the
agent could only get detached results via tasks check. Notes now carry
metadata source=task_note (kept by build) and is_display=False (clients
already show live task_update events). Verified live: agent reads the
note verbatim without calling any tool.

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