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/render.ts

@@ -163,6 +163,12 @@ function buildInstructionText(
 ): string {
   const marker = markerText(maxBytes, omitted, truncated)
   const body = [marker, style.intro, ...files.map(file => style.section(file))].filter(block => block.length > 0)
+  // Caller-owned framing: the plugin bakes the complete `<system-reminder>`
+  // frame into the message content. The session surface projects context
+  // verbatim and does not wrap it, so any framing must live here in the
+  // producer's content (the pattern a future `meta`-driven renderer would
+  // generalize — see the deferred note in
+  // ../../../../.agents/notes/implemented/simplification/2026-07-20-unwrap-injected-content-envelopes.md).
   return [SYSTEM_REMINDER_OPEN, body.join('\n\n'), SYSTEM_REMINDER_CLOSE].join('\n')
 }