Merge worktree-hooks-d-subagent into worktree-hooks-e-protocol

Bring the hook-protocol library branch onto the updated stack (master via A→B→C→D).
No review fix on E (#123 converged clean in its own round). The only conflict was
docs/rfc/README.md: kept D's corrected subagent RFC title (agentType dropped)
alongside E's own hook-protocol RFC index row.

docs/core-data-structures/session.md

@@ -80,7 +80,7 @@ type SessionEvent<T extends SessionEventType = SessionEventType> = {
     /**
      * Seq numbers of events that are provenance sources of this event
      * (e.g. the `assistant/chunk` seqs that built an `assistant/message`,
-     * or the surface nodes shadowed by a compaction marker).
+     * or the surface nodes shadowed by a compaction replace node).
      */
     sourceEventSeqs?: number[]
     /** How this event entered the surface; absent for non-surface events. */