Timmy companion: fail-closed hostile JSON, structured deltas, source_issue validation, bytecode cleanup

Addresses the four release-critical blockers from Timmy's review of PR #42:
1. Guard enum/allowlist membership against unhashable JSON (lists/dicts) in
   form.state, receipt.signed_by, and validate_transition target.
2. Require audience_choice.source_issue to be a positive non-bool integer.
3. Make power/scar/cost deltas structured (before -> after), not prose.
4. Remove tracked .pyc bytecode; prove clean checkout stays clean after tests.

Adds 21 new negative tests across HostileJsonFailClosedTests,
SourceIssueValidationTests, and StructuredDeltaTests.

Based on PR #42 head (0b74302). Vincent retains full authorship of the
canon validator, lifecycle model, and r2 review response; this branch
contains only the integration fixes needed to make the stated security
contract true at hostile-input boundaries.

Refs: #19, PR #42

scripts/lab_loop.py

@@ -68,11 +68,16 @@ class Gitea:
         if missing:
             raise LoopError(f"Missing repository labels: {', '.join(missing)}")
         self.request(
-            "PATCH",
-            f"/repos/{self.repo}/issues/{number}",
+            "PUT",
+            f"/repos/{self.repo}/issues/{number}/labels",
             {"labels": [label_map[name] for name in sorted(names)]},
         )
 
+    def set_assignee(self, number: int, agent: str) -> None:
+        if agent not in AGENTS:
+            raise LoopError(f"Unsupported assignee: {agent}")
+        self.request("PATCH", f"/repos/{self.repo}/issues/{number}", {"assignee": agent})
+
     def comment(self, number: int, body: str) -> None:
         if not body.strip():
             raise LoopError("Refusing to post an empty comment")
@@ -163,6 +168,7 @@ def cmd_handoff(api: Gitea, agent: str, args: argparse.Namespace) -> int:
     body = read_body(args, f"Continue the bounded task in issue #{args.number}.")
     api.comment(args.number, f"[HANDOFF] from={agent} to={args.to}\n\n{body.strip()}")
     api.set_labels(args.number, transition(names, agent=args.to, state="ready"))
+    api.set_assignee(args.number, args.to)
     verified = api.issue(args.number)
     validate_owner(verified, args.to, "ready")
     print(json.dumps({"status": "handed_off", "number": args.number, "from": agent, "to": args.to}))