fix(loop): record a durable prompt/blocked for every vetoed prompt

Address review on the interception-seams PR: PromptDecision.reason is
documented as the durable record of why a prompt was blocked, but the loop
only surfaced it via the fully-blocked batch's `rejected` turn/end. In a MIXED
batch — one queued prompt blocked, another allowed — the turn does not end
`rejected`, so the blocked prompt and its reason vanished from the session log
entirely.

Add a `prompt/blocked` SessionEventMap variant (content + source + reason),
appended in the open turn at the veto point in place of the user/message the
prompt would have become. It is a non-surface, turn-enclosed event (like
todo/write): it never reaches deriveMessages(). The fully-blocked batch still
also ends `rejected` for boundary balance + ACP settlement. Regression test
drives a mixed batch and asserts the blocked prompt is recorded while the
allowed one runs — proven red without the append.

packages/core/agent-loop/src/loop.ts

@@ -385,6 +385,14 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
       )
       if (decision.kind === 'block') {
         lastBlockReason = decision.reason
+        // Record the veto durably: `PromptDecision.reason` is the durable record
+        // of why a prompt was blocked, but a fully-blocked batch's `rejected`
+        // turn/end only preserves the LAST reason, and a MIXED batch (this prompt
+        // blocked, another allowed) does not end `rejected` at all — so without
+        // this append a blocked prompt would vanish from the log whenever any
+        // sibling prompt is allowed. `prompt/blocked` sits in the open turn in
+        // place of the `user/message` this prompt would have become.
+        session.append('prompt/blocked', { content: message.content, source: message.source, reason: decision.reason })
         continue
       }
       anyAllowed = true