subagents: a stopped run closes the tool it had opened ...

A tool card is drawn from tool_started and only its tool_call closes it.
When the user stopped a run while a sub-agent was mid-tool, the sub-agent
was cancelled, the parent dropped its sink, and the matching tool_call was
never sent — the card spun until the page was reloaded.

The sub-agent now reports its in-flight tool from the CancelledError
handler, and both batch paths (sequential and parallel) drain the sink
after cancelling instead of discarding it, so that event reaches the
client ahead of the parent's synthetic "stopped by the user" result.

The webclient settles anything still pending at stream_end,
stream_stopped and error as a safety net for a lost frame or a dying
server, with a visible "interrupted" note rather than a silent close.

The TUI needs nothing: it renders tool_call as a result panel and carries
no pending state, so it gets the fix through the backend alone.

switch_profile: run the switch alone, then the batch on the new tools ...

A tool called in the same batch as switch_profile was still dispatched against
the old tool_map and died with "tool 'X' not found" — reload_tools did, twice
in one session. The batch is now split: the switch runs first, its
ProfileSwitched event is watched for the target profile, the tools are
re-resolved from it, and the remaining calls run against the new set.

The turn's own profile binding is deliberately left alone — the
end-of-iteration reload in run_stream() is what rebinds profile/llm/schemas,
and pre-setting session.profile_id here would make it skip that step.

background tasks: review fixes B1-B11 batch ...

- stop mid-batch cancels in-flight tools (B1) and keeps real results
  of already-finished ones, mixing them with synthetic stopped notes
  in call order (B2)
- queued messages: headless drain publishes session_sync (B4), the
  run's teardown broadcasts session_sync to other sockets but not the
  owner socket (prevents double reload) (B5)
- task_update notifications are chained per task so late running
  updates can't overtake the terminal one (B7; client drops late
  re-flicker of a terminal task chip) (B8)
- client: ui_component results reference the owning message via
  card.parentMsg instead of a stale msg reference (B6)
- tasks cancel reports the real outcome after a bounded wait instead
  of an optimistic 'cancelled' (B9)
- session delete cancels its running background jobs and drops
  pending result notes (B10)
- subagent tool loop routes background:true calls through
  ToolExecutor._maybe_background like the main loop (B11)
- tests for all of the above; docs/tasks.md stop/cancel semantics

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