refactor agent pre-step inbox lifecycle

.agents/notes/implemented/feature/2026-07-21-cross-session-references.md

@@ -26,9 +26,9 @@ One aggregated context is serialized as JSON beneath a fixed untrusted-backgroun
 
 ## Message ownership
 
-TUI owns the snapshot/direct-message transaction without extending the generic inbox record. Outside the next-step acceptance window, it installs a one-shot outer `agent/prompt-submit` listener before `followup()`; an allowed decision receives the snapshot as `additionalContexts`, while a blocked or discarded prompt releases the listener and writes neither message. During prompt admission or an open turn, TUI calls `inject(snapshot)` then `steer(prompt)`, and AgentLoop stages both for the same safe boundary. If admission fails before that boundary, both remain staged for retry or a later admitted prompt; cancellation or disposal may discard them. The [separate-context decision](../architecture/2026-07-24-separate-context-injection-from-turn-execution.md) owns this generic delivery boundary.
+TUI owns the snapshot/direct-message transaction without extending the generic inbox record. While the agent is idle, it installs a one-shot outer `agent/pre-step` listener before `followup()`; an enter decision receives the snapshot as another message, while rejection or an earlier ordinary discard releases the listener and writes neither message. While the agent is running, TUI calls `inject(snapshot)` then `steer(prompt)`, placing both in the next-step inbox for the same later claim. A rejecting or failed pre-step leaves that claimed pair removed; messages inserted after the claim remain pending. The [separate-context decision](../architecture/2026-07-24-separate-context-injection-from-turn-execution.md) owns this generic delivery boundary.
 
-Reference preparation is not a new steering protocol and does not create a turn by itself. A `followup()` outside the next-step acceptance window dispatches prompt admission; steering inside the window bypasses it while retaining snapshot order through the shared outbox.
+Reference preparation is not a new steering protocol and does not create a turn by itself. Idle delivery uses `followup()` and pre-step entry; running delivery uses the shared next-step inbox while retaining snapshot order.
 
 ## Host adapters
 
@@ -45,8 +45,8 @@ Each of at most three references is independently capped at 65,536 UTF-8 bytes b
 - **Wait for SQLite FTS5** — rejected because snapshot correctness requires exact id reads and canonical surface folding, not content search. FTS improves discovery only.
 - **Put mention syntax in agent delivery methods** — rejected because it would make the core protocol parse one host's presentation syntax and prevent typed non-text hosts from sharing the semantic layer.
 - **Implement references separately in each host** — rejected because projection, security warning, retention, and persistence would drift across hosts.
-- **Attach context to `SendOptions` and the inbox record** — rejected because generic delivery would own a domain transaction through admission, steering, cancellation, and observation. A domain-specific admission wrapper and the existing next-step outbox preserve the required pairing without enlarging every message.
-- **Bake the prefix host-side before `followup()`** — rejected because `agent/prompt-submit` must inspect and rewrite only the direct prompt. Keeping the snapshot as a separate sourced message preserves that boundary and lets TUI hide background bytes from the direct user bubble.
+- **Attach context to `SendOptions` and the direct prompt's inbox record** — rejected because generic delivery would own a domain transaction through admission, steering, cancellation, and observation. A domain-specific admission wrapper and the existing next-step inbox preserve the required pairing without enlarging every direct prompt.
+- **Bake the prefix host-side before `followup()`** — rejected because `agent/pre-step` must inspect and rewrite only the direct prompt. Keeping the snapshot as a separate sourced message preserves that boundary and lets TUI hide background bytes from the direct user bubble.
 - **Replay the raw source log or restore shadowed events** — rejected because compact defines the current model surface and may intentionally retire sensitive or expensive history.
 - **Resume or fork the source** — rejected because the feature supplies read-only background for one target message, not identity or lifecycle continuity.
 - **Inject at request time by rereading the source** — rejected because the reference would become nondeterministic, cancellation races could alter its bytes, and target replay would depend on external mutable state.