Fix subagent in-process result scoping (Codex review round 1)

Two merge-blocking bugs in the shared in-process run driver, both rooted in
`readResult` scanning the whole child session and deriving the stop reason only
from `turn/end`:

- A pre-turn `cancel()` cleared the queued prompt before any `turn/end` was
  logged, so the run settled `error` instead of `aborted`, violating the
  `SubagentRun.cancel()` contract. The driver now tracks that a cancel was
  requested and maps the no-turn case to `aborted`.
- A fork child whose own turn produced no `assistant/message` returned the
  SEEDED parent's last message as a `completed` success. `readResult` now scopes
  to the child's OWN events (after the seed prefix), so a message-less child
  yields empty output.

Both fixes carry a regression test proven to go red on the pre-fix driver.

Also: correct the `SubagentRun.id` / event-payload docs (it is the child AGENT
id, not a session id — the backend mints distinct tokens); refresh the stale
`coding-agent` welcome string (subagent is now a tool); and replace the stale
`TODO(sub-agents)` "deferred" prose in the Agent interface, core.md, and
architecture.md with an accurate pointer to the realized seam.

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

@@ -128,6 +128,22 @@ describe('dsh-subagent-spawn', () => {
     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
+    // `turn/end` alone then mis-maps the no-turn case to `error`; the run must
+    // honor the cancel contract and settle `aborted`. The cancel is synchronous
+    // (same tick as start, before the loop's queued-wait continuation runs), so
+    // the turn is dropped and the empty script is never consumed.
+    const { ctx, parent } = await setup([])
+    const run = ctx.subagents.start('spawn', { prompt: [{ type: 'text', text: 'p' }], parent })
+    run.cancel('early')
+    const result = await run.result
+    expect(result.stopReason).toBe('aborted')
+    expect(result.output).toEqual([])
+    await run.dispose()
+  })
+
   it('cancelling a running child settles the run as aborted (the abort bridge + cancel())', async () => {
     // 'hang' makes the child's model stream one chunk then wait until aborted.
     const controller = new AbortController()