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/cordis-catalog/events-and-services.md

@@ -25,7 +25,7 @@ An agent was registered in the AgentRegistry and is ready to receive messages.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:136`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:137`](../../packages/core/agent/src/types.ts)
 
 #### `agent/disposed` — emit
 
@@ -37,7 +37,7 @@ An agent was disposed and removed from the registry; its fiber and any in-flight
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:142`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:143`](../../packages/core/agent/src/types.ts)
 
 #### `agent/error` — emit
 
@@ -49,7 +49,7 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:219`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:220`](../../packages/core/agent/src/types.ts)
 
 #### `agent/queued` — emit
 
@@ -61,7 +61,7 @@ A message entered the agent's inbox (queued or steering). `source` is the resolv
 
 Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:155`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:156`](../../packages/core/agent/src/types.ts)
 
 #### `agent/request` — waterfall
 
@@ -73,7 +73,7 @@ Waterfall: mutate the fully-assembled GenerateOptions before the model call (hoo
 
 Types: [Agent](../core-data-structures/core.md) · [GenerateOptions](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:188`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:189`](../../packages/core/agent/src/types.ts)
 
 #### `agent/status` — emit
 
@@ -85,7 +85,7 @@ Agent status changed (`idle` ⇄ `running`, or → `disposed`). Drive lifecycle
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:149`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:150`](../../packages/core/agent/src/types.ts)
 
 #### `agent/steering` — emit
 
@@ -97,7 +97,7 @@ Steering content was injected into a running turn.
 
 Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:213`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:214`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-end` — emit
 
@@ -109,7 +109,7 @@ A step ended.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:179`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:180`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-result` — waterfall
 
@@ -121,7 +121,7 @@ Waterfall: post-process the assembled assistant Message before tool dispatch (va
 
 Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:194`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:195`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-start` — emit
 
@@ -133,7 +133,7 @@ A step (one model call plus its tool dispatch) began. `step` is 1-based within t
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:174`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:175`](../../packages/core/agent/src/types.ts)
 
 #### `agent/stream-chunk` — emit
 
@@ -145,7 +145,7 @@ A raw StreamChunk arrived from the model (token-level UI/log feed).
 
 Types: [Agent](../core-data-structures/core.md) · [StreamChunk](../core-data-structures/llm-streaming.md)
 
-Source: [`packages/core/agent/src/types.ts:208`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:209`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-continuation` — waterfall
 
@@ -157,7 +157,7 @@ Waterfall: override the turn-continuation decision. The default (computed by the
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:201`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:202`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-end` — emit
 
@@ -169,7 +169,7 @@ A turn ended. `reason` distinguishes a clean stop from a truncated or aborted on
 
 Types: [Agent](../core-data-structures/core.md) · [TurnEndReason](../core-data-structures/session.md)
 
-Source: [`packages/core/agent/src/types.ts:168`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:169`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-start` — emit
 
@@ -181,7 +181,7 @@ A turn began. `turn` is the 1-based turn number within the session.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:162`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:163`](../../packages/core/agent/src/types.ts)
 
 ### `llm/*`