fix(session): project steering messages as plain user content

steering/message previously rendered inside a <steering source="…">
envelope like context/message. No model is trained on a <steering> tag,
so the framing is arbitrary markup the model was never taught to read;
recorded transcripts show it treating the instruction as third-party
metadata and refusing it. Framing also does not belong on the session
surface — a caller that wants a frame formats its own content. It now
projects verbatim as a plain user-role message; the <context> envelope is
untouched and renderTagged becomes context-only.

Agent Note: .agents/notes/implemented/simplification/2026-07-20-unwrap-steering-message-projection.md

docs/cordis-catalog/services.md

@@ -820,7 +820,7 @@ fork(source: SessionForkSource, boundary?: number, childSessionId?: SessionId):
 
 Types: [CreateSessionOptions](../core-data-structures/persistence.md) · [Session](../core-data-structures/session.md) · [SessionId](../core-data-structures/core.md)
 
-Source: [`packages/core/session/src/index.ts:577`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:574`](../../packages/core/session/src/index.ts)
 
 ## `ctx.skills` — `SkillService`