refactor(vocab): prune producer-less variants — cache hints, agent source, continuation trigger

The vocabulary maps grow by declaration merging, and the admission policy
stated on TurnEndReasonMap is that a variant lands with its first emitter.
Three declared items had no producer and no consumer:

- CacheHint and the cache?: CacheHint fields on TextBlock/ToolResultBlock:
  nothing constructs a block with cache:, and neither adapter reads .cache —
  DeepSeek prompt caching is automatic (hints map OUT of responses, never IN).
- MessageSourceMap.agent: zero constructors; the subagent backends send the
  parent prompt with no source (logs as user), and the envelope renderer
  interpolates source.kind without routing on it.
- TurnTriggerMap.continuation: the loop structurally cannot emit it —
  continuation is further steps within a turn, never a new turn — and its only
  writer was an llm-replay test fixture that needed any non-message trigger
  (now an injection trigger).

Each variant returns the day it gains a real producer, via the same
merge-extensible maps. Docs updated in the same change: the MessageSourceMap
paste in core.md, the TurnTriggerMap paste in session.md (manifest untouched —
both symbols survive), the content-block vocabulary RFC's cache-hints
consequence line, and the RFC moved to implemented/ and amended to shipped
reality (the image block's own cache field had already left with the
drop-image RFC).

packages/support/llm-replay/tests/llm-replay.spec.ts

@@ -118,7 +118,7 @@ describe('deriveReplayScript', () => {
   it('ignores non-assistant/chunk events', () => {
     let seq = 1
     const events: SessionEvent[] = [
-      { type: 'turn/start', seq: seq++, time: 0, data: { turn: 1, trigger: { kind: 'continuation' } } },
+      { type: 'turn/start', seq: seq++, time: 0, data: { turn: 1, trigger: { kind: 'injection', source: { kind: 'user' } } } },
       ...TEXT_CHUNKS.map(c => chunkEvent(seq++, 1, 1, c)),
       { type: 'turn/end', seq: seq++, time: 0, data: { turn: 1, reason: { kind: 'completed' } } },
     ]