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/src/state.ts

@@ -61,9 +61,8 @@ export interface ReconciledInstructionContext {
   versionUpdates: InstructionVersionUpdate[]
 }
 
-/** Plugin-owned raw context with required replay metadata. */
+/** Plugin-owned context with required replay metadata. */
 export interface WorkspaceHookContext extends HookContext {
-  envelope: 'raw'
   meta: JsonValue
 }
 
@@ -76,7 +75,7 @@ function workspaceContextHook(text: string, changes: WorkspaceInstructionChange[
     ...change.digest !== undefined ? { digest: change.digest } : {},
   }))
   const meta: JsonValue = { kind: 'workspace-instructions', version: 1, changes: serializedChanges }
-  return { content: [{ type: 'text', text }], source: PLUGIN_SOURCE, envelope: 'raw', meta }
+  return { content: [{ type: 'text', text }], source: PLUGIN_SOURCE, meta }
 }
 
 /**