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.

docs/core-data-structures/core.md

@@ -273,11 +273,12 @@ interface Agent {
    */
   whenIdle(): Promise<void>
 
-  // TODO(sub-agents): spawn/fork seams — semantics deliberately deferred.
-  // The intended shape: a creation option referencing a parent agent
-  // (fork = seed the child Session with the parent's event log; spawn =
-  // fresh Session), with the child returned as an Agent handle so steer()
-  // and event subscription work uniformly. See docs/architecture.md.
+  // Subagent delegation is realized on top of this interface by the
+  // `@deepseek-ai/dsh-subagent` seam, not by a method here: a backend creates
+  // the child through `ctx.agents.create` (fork seeds the child Session with a
+  // balanced prefix of the parent's log via `CreateAgentOptions.seed`; spawn
+  // starts fresh) and drives it as an ordinary Agent handle, so steer() and
+  // event subscription work uniformly. See docs/core-data-structures/subagent.md.
 }
 ```