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/tools/tests/code-mode.spec.ts

@@ -487,7 +487,6 @@ describe('the run_code dispatch bridge', () => {
           additionalContexts: [{
             content: [{ type: 'text' as const, text: `context for ${exec.callId}` }],
             source: { kind: 'plugin' as const, plugin: 'test' },
-            envelope: 'raw' as const,
             meta: { callId: exec.callId },
           }],
         })
@@ -505,13 +504,11 @@ describe('the run_code dispatch bridge', () => {
       {
         content: [{ type: 'text', text: 'context for call-1:code:1' }],
         source: { kind: 'plugin', plugin: 'test' },
-        envelope: 'raw',
         meta: { callId: 'call-1:code:1' },
       },
       {
         content: [{ type: 'text', text: 'context for call-1:code:2' }],
         source: { kind: 'plugin', plugin: 'test' },
-        envelope: 'raw',
         meta: { callId: 'call-1:code:2' },
       },
     ])