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/hooks/hooks-codex/tests/coverage-cases.ts

@@ -125,7 +125,6 @@ export function defineCoverageCases(groups: CoverageGroup | readonly CoverageGro
         additionalContexts: [{
           content: [{ type: 'text' as const, text: 'from-downstream' }],
           source: { kind: 'plugin' as const, plugin: 'policy' },
-          envelope: 'raw' as const,
           meta: { owner: 'policy' },
         }],
       }))
@@ -140,7 +139,6 @@ export function defineCoverageCases(groups: CoverageGroup | readonly CoverageGro
         { kind: 'plugin', plugin: 'hooks-codex' },
         { kind: 'plugin', plugin: 'policy' },
       ])
-      expect(contexts[1]?.type === 'context/message' && contexts[1].data.envelope).toBe('raw')
       expect(contexts[1]?.type === 'context/message' && contexts[1].data.meta).toEqual({ owner: 'policy' })
     })
   })
@@ -171,7 +169,6 @@ export function defineCoverageCases(groups: CoverageGroup | readonly CoverageGro
         additionalContexts: [{
           content: [{ type: 'text' as const, text: 'downstream-note' }],
           source: { kind: 'plugin' as const, plugin: 'policy' },
-          envelope: 'raw' as const,
           meta: { owner: 'policy' },
         }],
       }))
@@ -183,7 +180,6 @@ export function defineCoverageCases(groups: CoverageGroup | readonly CoverageGro
         { kind: 'plugin', plugin: 'hooks-codex' },
         { kind: 'plugin', plugin: 'policy' },
       ])
-      expect(contexts[1]?.type === 'context/message' && contexts[1].data.envelope).toBe('raw')
       expect(contexts[1]?.type === 'context/message' && contexts[1].data.meta).toEqual({ owner: 'policy' })
     })