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/llm/llm/src/types.ts

@@ -22,14 +22,10 @@
 import type { Branded } from '@deepseek-ai/dsh-brand'
 import type { CallId } from './brand.ts'
 
-/** Cache hint attached to a content block (provider-interpreted). */
-export type CacheHint = 'ephemeral'
-
 /** Plain text visible to the end user. */
 export interface TextBlock {
   type: 'text'
   text: string
-  cache?: CacheHint
 }
 
 /** Reasoning / thinking content, distinct from visible text. */
@@ -54,7 +50,6 @@ export interface ToolResultBlock {
   toolCallId: CallId
   content: ContentBlock[]
   isError?: boolean
-  cache?: CacheHint
 }
 
 /**
@@ -91,7 +86,6 @@ export interface Message {
 export interface MessageSourceMap {
   user: { kind: 'user' }
   plugin: { kind: 'plugin'; plugin: string }
-  agent: { kind: 'agent'; agentId: string }
 }
 
 export type MessageSource = MessageSourceMap[keyof MessageSourceMap]