fix(tool-goal): let the model deliver a wrap-up message after a goal-round complete/blocked

A goal round reporting complete or blocked used to conclude the turn at
the tool result, so the model never spoke after the call and sessions
ended on a bare update_goal card. The terminal update now defers one
plugin-sourced <goal_complete>/<goal_blocked> instruction onto its
result asking for a grounded closing message without further tool
calls; the turn then ends through the ordinary no-tool-calls stop.
Direct-human mutations stay uninstructed. Wording chosen by A/B
sampling on deepseek-v4-pro; one extra request per goal lifecycle.

New keyless ACP snapshot goal-wrapup drives the shipped app through
create -> round one -> autonomous complete and pins the injection, the
same-turn closing message, and the completed turn end.

examples/acp-agent/tests/goal-snapshots/goal-wrapup/session.jsonl

@@ -0,0 +1 @@
+{"type":"session","version":0,"id":"goal-wrapup-placeholder","createdAt":0,"cwd":"{{cwd}}"}