fix(story-mode): a stop says where it landed and what was saved before it

From review round 8: "STOPPED" with the balloon "Step 3: new tests under
test/." read like progress, and nothing said what the stop left. The
final panel now names the step Claude last announced and the commits made
before the stop: "You stop Claude here, during its step 3. Saved before
the stop: 2 commits."

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/fixtures/story-corpus/teammates-then-interrupt.expected.json

@@ -300,7 +300,7 @@
       {
         "kind": "complete",
         "ok": null,
-        "story": "You stop Claude here.",
+        "story": "You stop Claude here, during its step 3. Saved before the stop: 2 commits.",
         "director": "interrupted · 2 min 41 s · 5 tool calls · 4 files changed",
         "balloon": "Step 3: new tests under test/.",
         "tests": null,