Honor an already-aborted request signal in the subagent driver (Codex review round 2)

A request signal aborted BEFORE the run starts never fires an `abort` event
(`addEventListener` only fires on the transition), so the backend-level bridge
missed it and ran the child to `completed`. The driver now checks
`request.signal?.aborted` at the top of the result path and settles `aborted`
without running the child. Regression test proven red on the pre-fix code.

Also refresh two stale RFC prose blocks the round-1 fix left behind: the
subagent RFC's Problem statement (cited the removed `TODO(sub-agents)` markers
and claimed nothing existed yet) and the unify-id RFC's fork/spawn risk bullet
(described the seam as "explicitly deferred" via `AgentLoop.create`'s old TODO),
now pointing at the realized seam.

packages/subagent/subagent-spawn/src/in-process.ts

@@ -143,6 +143,11 @@ export function startInProcessRun(
 
   const result: Promise<SubagentResult> = (async () => {
     try {
+      // A signal already aborted BEFORE the run starts never fires an `abort`
+      // event (`addEventListener` only fires on the transition), so the listener
+      // above won't catch it — settle `aborted` without running the child rather
+      // than completing an already-cancelled request.
+      if (request.signal?.aborted) return { output: [], stopReason: 'aborted' }
       child.send(request.prompt)
       await child.whenIdle()
       return readResult(child, seedLength, cancelled)

packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts

@@ -128,6 +128,22 @@ describe('dsh-subagent-spawn', () => {
     await run.dispose()
   })
 
+  it('settles aborted (without running the child) when the request signal is ALREADY aborted', async () => {
+    // Regression: a signal aborted BEFORE the run starts never fires an `abort`
+    // event, so the listener can't catch it. The driver must check the
+    // already-aborted case up front and settle `aborted` without running the
+    // child — otherwise an already-cancelled request runs to `completed`. The
+    // empty script proves the child's model is never called.
+    const controller = new AbortController()
+    controller.abort()
+    const { ctx, parent } = await setup([])
+    const run = ctx.subagents.start('spawn', { prompt: [{ type: 'text', text: 'p' }], parent, signal: controller.signal })
+    const result = await run.result
+    expect(result.stopReason).toBe('aborted')
+    expect(result.output).toEqual([])
+    await run.dispose()
+  })
+
   it('cancelling BEFORE the child turn starts settles aborted, not error', async () => {
     // Regression: a cancel landing in the pre-turn window clears the queued
     // prompt before any `turn/end` is logged. Deriving the stop reason from