Plugin API: stop a session until it is given work again (session.stop) #63650

The fork carries it as branch subagent-stop
(cevr/opencode, commit 4cb2aea off prime-look eb39956), and the dotfiles subagents tool's stop uses it.

What a plugin can do today

ctx.session.interrupt({ sessionID }) ends the session's current turn. For a subagent session the host then settles
it as cancelled: SubagentReport.prepare resolves an interrupted outcome to { status: "cancelled" }, cancels the
subagent's own Job, and delivers one <subagent state="cancelled"> notice to the parent. That part works.

What it cannot do

• The child's background work keeps running. Job.cancelSession(sessionID) runs only inside Session.remove,
  and it also deletes the child's own subagent job row (its report). A stopped child's background shell, when it
  ends, admits its notice to the child and wakes it. The child runs a new turn, and when it goes idle its parent gets
  a second report, state="completed": the parent is woken as if the child finished after it was stopped.
• Queued input stays parked. interrupt without resume leaves queued prompts in the inbox, and the plugin
  SessionDomain has no inbox; the next wake runs them.
• Any later wake reports again. A notice (a shell's end, a grandchild's report) or the owner prompting the child
  starts a period whose end reports to the parent.

The fork's change (proposed upstream)

• Job.cancelOwned(sessionID): cancels, as cancel does one, each running job the session owns (its shells,
  images, its own subagents' jobs), not its own subagent job.
• SessionExecution.stop(sessionID): marks the session stopped (KV session.stopped:<id>, durable across a
  restart), cancels its owned jobs, interrupts its turn and waits for it to settle. SessionExecution.stopped(id).
  A resume (the subagent tool giving the child a new task) clears the mark.
• While a session is stopped: Session.synthetic, a subagent's report to it, and the restart sweep do not wake it;
  and SubagentReport.prepare reports nothing to its parent for a period no Job promised a report for (the stop's
  own cancelled settlement is promised by the subagent job, so it is the one notice).
• Session.stop(sessionID): the execution stop, then cancels its inbox rows.
• Plugin (effect) SessionDomain.stop({ sessionID }) => { interrupted }. The promise API is not extended.

Tests: packages/core/test/subagent-report.test.ts "a stop cancels the child's background work, reports cancelled
once, and silences the child until a resume"; dotfiles tests/kernel.test.ts "a cell's subagent handle stops a real
child mid-turn…" (a real child with a running background shell and a queued prompt).

Alternatives considered

Exposing the pieces (session.inbox, a job cancel by session, a report-suppress hook, a wake gate) leaves the
policy in each plugin and needs four hooks where one operation keeps it in the host.