docs: audit package limitations on current stack

packages/core/agent/README.md

@@ -55,9 +55,8 @@ The handle every plugin programs against:
 ## Known Limitations and Deferred Work
 
 - **Inter-agent channels beyond delegation** — shared state, streaming child output, and background/poll semantics remain outside the current synchronous `ctx.subagents` seam.
-- **`agent/session-start` cannot gate startup** — a synchronous, veto-less emit, so an async listener's injection is best-effort before turn 1; startup gating is a deferred loop-level change.
+- **`agent/session-start` cannot gate startup** — it remains a synchronous, veto-less notification; async composition that must finish before publication belongs in the factory's `setup(agentCtx)` transaction instead.
 - **No public step-only abort** — `cancel()` clears ALL pending work (queued + steering + in-flight); an abort that preserves queued prompts returns only with a named consumer ([stop-surface RFC](../../../docs/rfc/implemented/simplification/2026-06-20-public-agent-stop-surface.md)).
-- **`AgentRegistry` enforces unique agent ids, not session ids** — two live agents sharing a session id mis-route bash owner-token notices; [id unification stays proposed](../../../docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md).
 - **`HookContext` carries exactly one `MessageSource`** — contributions from several plugins merged onto one tool call collapse under one source; mixed provenance is unrepresentable.
 - **`SessionStartSource` reserves `'clear'`/`'compact'` with no emitter yet** — only `'startup'`/`'resume'` occur until the driving subsystems land (`TODO(compaction)`).
 - **`agent/pre-step`'s `fullSystemPrompt`/`sessionPrefix` parameters are a flagged smell** — compaction is their only consumer; a lazy prompt provider or a compaction-specific pressure seam is the marked revisit.