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/agent-loop/tests/interception.spec.ts

@@ -99,7 +99,6 @@ describe('agent/prompt-submit', () => {
         additionalContexts: [{
           content: [{ type: 'text', text: '<system-reminder>extra ctx</system-reminder>' }],
           source: { kind: 'plugin', plugin: 'test' },
-          envelope: 'raw',
           meta,
         }],
       }))
@@ -113,7 +112,6 @@ describe('agent/prompt-submit', () => {
     expect(userMsg).toBeDefined()
     expect(ctxMsg?.type === 'context/message' && ctxMsg.data.content).toEqual([{ type: 'text', text: '<system-reminder>extra ctx</system-reminder>' }])
     expect(ctxMsg?.type === 'context/message' && ctxMsg.data.source).toEqual({ kind: 'plugin', plugin: 'test' })
-    expect(ctxMsg?.type === 'context/message' && ctxMsg.data.envelope).toBe('raw')
     expect(ctxMsg?.type === 'context/message' && ctxMsg.data.meta).toEqual(meta)
     const sent = JSON.stringify(adapter.requests[0]!.messages)
     expect(sent).toContain('extra ctx')
@@ -556,7 +554,6 @@ describe('tool additionalContexts buffering across a step', () => {
         additionalContexts: [{
           content: [{ type: 'text', text: `ctx-${exec.callId}` }],
           source: { kind: 'plugin', plugin: 'p' },
-          envelope: 'raw',
           meta: { callId: exec.callId },
         }],
       }))
@@ -580,7 +577,6 @@ describe('tool additionalContexts buffering across a step', () => {
       .map(b => (b.type === 'text' ? b.text : ''))
     expect(ctxTexts).toEqual(['ctx-c1', 'ctx-c2'])
     const contextEvents = events(agent).filter(e => e.type === 'context/message')
-    expect(contextEvents.map(e => e.type === 'context/message' && e.data.envelope)).toEqual(['raw', 'raw'])
     expect(contextEvents.map(e => e.type === 'context/message' && e.data.meta)).toEqual([{ callId: 'c1' }, { callId: 'c2' }])
   })
 
@@ -591,7 +587,7 @@ describe('tool additionalContexts buffering across a step', () => {
       name: 'composite', description: 'composite', parameters: {},
       async execute(_args, exec) {
         exec.deferContext({ content: [{ type: 'text', text: 'nested-a' }], source: { kind: 'plugin', plugin: 'a' }, meta: { order: 1 } })
-        exec.deferContext({ content: [{ type: 'text', text: 'nested-b' }], source: { kind: 'plugin', plugin: 'b' }, envelope: 'raw', meta: { order: 2 } })
+        exec.deferContext({ content: [{ type: 'text', text: 'nested-b' }], source: { kind: 'plugin', plugin: 'b' }, meta: { order: 2 } })
         return [{ type: 'text', text: 'outer result' }]
       },
     }))