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.
abe80cec68