fix: use exact issue-number boundary in own-branch adoption (#442)

Replace substring issue-marker matching with a numeric word-boundary
regex so issue-42 adoption is not false-blocked by unrelated issue-420
branches. Add regression tests for the #42 vs #420 collision.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

issue_lock_adoption.py

@@ -14,6 +14,8 @@ this module additionally records whether they passed for proof purposes.
 
 from __future__ import annotations
 
+import re
+
 ADOPT = "adopt_existing_branch"
 BLOCK_COMPETING = "block_competing_branch"
 NO_MATCH = "no_matching_branch"
@@ -33,6 +35,19 @@ def _branch_sha(entry) -> str | None:
     return None
 
 
+def _branch_carries_issue_marker(branch_name: str, issue_number: int) -> bool:
+    """Return True when *branch_name* references issue *issue_number* exactly.
+
+    Uses a numeric word-boundary so ``issue-42`` does not match inside
+    ``issue-420`` (AC6 / #440).
+    """
+    name = (branch_name or "").strip()
+    if not name:
+        return False
+    pattern = rf"(?:^|/)issue-{int(issue_number)}(?![0-9])"
+    return re.search(pattern, name) is not None
+
+
 def assess_own_branch_adoption(
     *,
     issue_number: int,
@@ -59,13 +74,12 @@ def assess_own_branch_adoption(
     BLOCK_COMPETING: at least one same-issue branch is not the requested branch.
     NO_MATCH: no branch carries the issue marker — normal lock path applies.
     """
-    marker = f"issue-{issue_number}"
     requested = (requested_branch or "").strip()
 
     matches: list[tuple[str, str | None]] = []
     for entry in existing_branches or []:
         name = _branch_name(entry).strip()
-        if marker in name:
+        if _branch_carries_issue_marker(name, issue_number):
             matches.append((name, _branch_sha(entry)))
 
     competing = sorted({name for name, _ in matches if name != requested})