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"
@@ -33,25 +35,58 @@ 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,
     requested_branch: str,
     existing_branches,
 ) -> dict:
-    """Decide whether an existing matching branch is adoptable."""
-    marker = f"issue-{issue_number}"
+    """Decide whether an existing matching branch is adoptable.
+
+    Args:
+        issue_number: The tracking issue number being locked.
+        requested_branch: The exact branch the caller wants to lock.
+        existing_branches: Iterable of remote branch entries — either names or
+            dicts with ``name`` and optional ``commit_sha``.
+
+    Returns:
+        dict with:
+          * ``outcome`` — one of ADOPT / BLOCK_COMPETING / NO_MATCH
+          * ``adopt`` (bool), ``block`` (bool)
+          * ``reason`` (str)
+          * ``matched_branch`` (str | None), ``matched_head_sha`` (str | None)
+          * ``competing_branches`` (list[str])
+
+    ADOPT: the issue's exact branch exists and no other same-issue branch does.
+    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.
+    """
     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})
     exact = [(name, sha) for name, sha in matches if name == requested]
 
+    # Fail closed whenever any non-requested same-issue branch exists, even if
+    # the requested branch is also present: ownership is then ambiguous.
     if competing:
         return {
             "outcome": BLOCK_COMPETING,
@@ -103,7 +138,12 @@ def build_adoption_proof(
     lock_file_path: str,
     lock_file_status: str,
 ) -> dict:
-    """Assemble the proof block returned by ``gitea_lock_issue`` on adoption."""
+    """Assemble the proof block returned by ``gitea_lock_issue`` on adoption.
+
+    Requirement #4: adoption results must carry issue number, branch name,
+    branch head commit, adoption reason, no-existing-PR proof, no-competing-
+    live-lock proof, and lock file path/status.
+    """
     return {
         "issue_number": issue_number,
         "branch_name": branch_name,