fix(events): address review — core.md turn-only taxonomy, RFC mechanism names, post-execute content snapshot

- core-data-structures/core.md: the `agent/*` taxonomy said "turn/step
  boundaries", but the step-boundary mirror emits were dropped — `agent/*`
  mirrors only turn boundaries; step boundaries are durable `step/start`/
  `step/end` session events. Narrow the catalog so plugin authors aren't pointed
  at nonexistent `agent/*` step events.
- interception-seams RFC: replace stack-position phrasing ("a later stack PR",
  "the stack's first change", "the PR that makes...") with durable mechanism/RFC
  names (the hook bridge packages, the event-domain-semantics RFC).
- tools/post-execute snapshot: `dispatched.content` was the same array reference
  as `result.content`, so a listener's in-place `push`/`splice` leaked into the
  returned content while a reassignment was masked — the "protect from tampering"
  comment over-claimed. Copy content into a fresh array so the snapshot guards
  the array structure; comment now states it is not deep immutability. Regression
  extended to push a block in-place and assert it does not leak (proven red
  without the copy).

docs/core-data-structures/core.md

@@ -306,7 +306,7 @@ interface Agent {
 }
 ```
 
-`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`, `systemPrompt?`) is merge-extensible — plugins add creation options by declaration merging. The `agent/*` event taxonomy (lifecycle, turn/step boundaries, the `agent/prompt-submit`/`agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy).
+`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`, `systemPrompt?`) is merge-extensible — plugins add creation options by declaration merging. The `agent/*` event taxonomy (lifecycle, live turn boundaries, the `agent/prompt-submit`/`agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy); step boundaries are durable `step/start`/`step/end` session events only — `agent/*` mirrors turn boundaries, not steps.
 
 ## Interception decisions