Make mid-run context injection a clean, first-class operation

[0xbeckett]

Problem

Running an agent as a service, then injecting / interrupting / adding context to that already-running agent, was unreliable. The live-steering path was fire-and-forget: nudge() wrote the steer to the live worker's stdin and returned "delivered" with no guarantee it was ever applied. A worker mid-turn could barrel past the new context — steers landed late or were silently skipped — and a delivered steer was never buffered, so a worker that ignored it, died, or retried lost it entirely.

Fix — steering is now deterministic, with two explicit modes

enqueue (nudge) — the steer is durably buffered and handed to the live worker best-effort. Every steer carries a stable id; a worker that picks it up echoes a nudge_ack, which drains the buffer. A worker that ignores it, dies, or retries leaves the steer buffered, so it folds into the next seat's brief. The steer is never silently skipped and never lost past the run — it lands either on the running worker (confirmed by ack) or, at latest, on the run's next seat. The in-flight seat keeps running.

interrupt (interrupt, new verb) — the steer is buffered, the in-flight seat is aborted with a WIP checkpoint, and the same visit is re-staffed (next attempt, no retry cap burned) with the steer folded into a fresh brief. Immediate, deterministic application to the current node's work; the run never leaves running.

Mechanics

• New run events: nudge_delivered.steerId + nudge_acked. The fold buffers any steer carrying an id (dedup by id) and drains on ack or on the next seat spawn. The buffer is a fold of the persisted log, so it survives an engine reboot.
• New WorkerEvent nudge_ack; the process adapter streams {kind,steerId,text} on stdin and parses the ack back off stdout. The Sentinel's liveness pokes carry no id and never fold into a brief.
• StageManager.interrupt(), plumbed through the tracker, HTTP API (mode: "enqueue" | "interrupt"), CLI (nudge … --interrupt), and flow-script hooks.

Interrupt-vs-enqueue, stated

	enqueue (nudge)	interrupt (interrupt)
in-flight seat	keeps running	aborted (WIP committed)
when applied	on ack, else next seat	now, on a fresh re-staffed seat
guarantee	never skipped, never lost past the run	applied to the current node's work
run status	unchanged	stays running

Tests

• test/steering.test.ts (new): enqueue durability + ack drain; an unacked steer surviving into the next seat's brief; buffering on a parked run and across an engine reboot; interrupt abort + same-visit re-staff carrying the steer.
• test/process-adapter.test.ts: asserts the nudge_ack round-trips over stdin and drains the buffer.
• Full suite green: 19 files / 176 tests.

Left unmerged for a human to review.

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