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/context/workspace-context/tests/workspace-context.spec.ts

@@ -173,7 +173,6 @@ function stubAgent(cwd?: string, seed: SessionEvent[] = []): Agent {
       session.append('context/message', {
         content,
         source: options?.source ?? { kind: 'user' },
-        ...options?.envelope !== undefined ? { envelope: options.envelope } : {},
         ...options?.meta !== undefined ? { meta: options.meta } : {},
       }, { surfaceOp: 'append' })
     },
@@ -202,7 +201,6 @@ function workspaceChangeContext(scope: string, digest: string): HookContext {
   return {
     content: [{ type: 'text', text: `instructions for ${scope}` }],
     source: { kind: 'plugin', plugin: 'workspace-context' },
-    envelope: 'raw',
     meta: {
       kind: 'workspace-instructions',
       version: 1,
@@ -217,7 +215,6 @@ function appendAdditionalContexts(agent: Agent, result: { additionalContexts?: H
     lastSeq = agent.session.append('context/message', {
       content: context.content,
       source: context.source,
-      ...context.envelope !== undefined ? { envelope: context.envelope } : {},
       ...context.meta !== undefined ? { meta: context.meta } : {},
     }, { surfaceOp: 'append' }).seq
   }
@@ -1695,7 +1692,6 @@ describe('dynamic nested workspace context injection', () => {
 
       expect(result.isError).toBe(false)
       expect(workspaceContextOf(result)?.source).toEqual({ kind: 'plugin', plugin: 'workspace-context' })
-      expect(workspaceContextOf(result)?.envelope).toBe('raw')
       expect(workspaceContextOf(result)?.meta).toMatchObject({
         kind: 'workspace-instructions',
         version: 1,
@@ -2461,7 +2457,6 @@ describe('dynamic nested workspace context injection', () => {
       expect(blocksText(result.content)).toBe('downstream replacement')
       expect(result.additionalContexts).toHaveLength(2)
       expect(workspaceContextOf(result)?.source).toEqual({ kind: 'plugin', plugin: 'workspace-context' })
-      expect(workspaceContextOf(result)?.envelope).toBe('raw')
       expect(workspaceContextOf(result)?.meta).toMatchObject({
         kind: 'workspace-instructions',
         changes: [{ action: 'set', scope: 'pkg', path: 'pkg/AGENTS.md' }],
@@ -2474,7 +2469,8 @@ describe('dynamic nested workspace context injection', () => {
       })
       const agent = stubAgent(root)
       appendAdditionalContexts(agent, result)
-      expect(blocksText(agent.session.deriveMessages()[1]?.content)).toContain('<context source="plugin">\ndownstream context\n</context>')
+      expect(blocksText(agent.session.deriveMessages()[1]?.content)).toContain('downstream context')
+      expect(blocksText(agent.session.deriveMessages()[1]?.content)).not.toContain('<context source=')
     } finally {
       await rm(root, { recursive: true, force: true })
       await rm(home, { recursive: true, force: true })
@@ -2807,7 +2803,6 @@ describe('workspace context pending state', () => {
     const otherWorkspaceEvent = agent.session.append('context/message', {
       content: otherContext.content,
       source: otherContext.source,
-      ...otherContext.envelope !== undefined ? { envelope: otherContext.envelope } : {},
       ...otherContext.meta !== undefined ? { meta: otherContext.meta } : {},
     }, { surfaceOp: 'append' })
     observeInstructionSessionEvent(agent.session, otherWorkspaceEvent, pending, versions)
@@ -2817,7 +2812,6 @@ describe('workspace context pending state', () => {
     const confirmed = agent.session.append('context/message', {
       content: context.content,
       source: context.source,
-      ...context.envelope !== undefined ? { envelope: context.envelope } : {},
       ...context.meta !== undefined ? { meta: context.meta } : {},
     }, { surfaceOp: 'append' })
     observeInstructionSessionEvent(agent.session, confirmed, pending, versions)