fix(session): drop the <context> envelope and project context verbatim

context/message previously defaulted to a <context source="…">…</context>
wrapper. No model is trained on a <context> tag either, and message
framing does not belong on the session surface: the surface projects the
durable log, and a caller that wants a frame formats its own content —
which the one heavy producer (workspace-context) already does with its own
<system-reminder> frame, opting out via 'raw'. The tag only added
machinery — ContextEnvelope plus an envelope field threaded through
InjectOptions, HookContext, the context/message event, and the agent-loop
inject/additionalContexts plumbing.

context/message now projects its content verbatim as a user-role message,
sharing one deriveEventMessage case with user/message and steering/message.
ContextEnvelope and every envelope field are removed; context/message.meta
still carries durable, model-hidden JSON state. Regenerated catalogs and
website API; refreshed the three affected keyless snapshots (envelope field
only; timestamps unchanged).

Broadens and renames the steering Agent Note to cover both envelope
removals as one decision.

Agent Note: .agents/notes/implemented/simplification/2026-07-20-unwrap-injected-content-envelopes.md

packages/core/session/tests/surface.spec.ts

@@ -369,7 +369,7 @@ describe('deriveMessages with surface', () => {
     s.append('steering/message', { turn: 1, content: [{ type: 'text', text: 'focus' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
     const messages = s.deriveMessages()
     expect(messages).toHaveLength(2)
-    expect(messages[0]!.content[0]).toMatchObject({ type: 'text', text: '<context source="plugin">' })
+    expect(messages[0]!.content).toEqual([{ type: 'text', text: 'file changed' }])
     expect(messages[1]!.content).toEqual([{ type: 'text', text: 'focus' }])
   })
 })