reuse_refs_check: sibling repos beside the MAIN clone for a worktree (R-1114)

FELHOM_WORKSPACE (set by felhom-controller's runner) names where the sibling repos are; the worktree's own
main clone is skipped. Unset: unchanged. Test red-proved both ways.

Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

scripts/reuse_refs_check.py

@@ -115,13 +115,33 @@ def index_for(root):
     return _index_cache[root]
 
 
+def _main_clone_of_worktree(root):
+    """The main clone of a linked worktree (its `.git` FILE reads `gitdir: <main>/.git/worktrees/<name>`), else None."""
+    try:
+        with open(os.path.join(root, ".git")) as fh:
+            line = fh.readline().strip()
+    except OSError:  # a directory (a main clone) or absent
+        return None
+    if not line.startswith("gitdir:"):
+        return None
+    gitdir = os.path.realpath(line[len("gitdir:"):].strip())
+    head, tail = os.path.split(os.path.dirname(gitdir))  # <main>/.git, "worktrees"
+    if tail != "worktrees" or os.path.basename(head) != ".git":
+        return None
+    return os.path.dirname(head)
+
+
 def find_siblings(root):
     """Immediate sibling dirs of <root> that are themselves git working trees. One level only.
 
     Returns (list_of_paths, error_message_or_None). A sibling repo that is simply absent is NEVER
     a failure — a clone in isolation must still be able to check itself.
     """
-    parent = os.path.dirname(os.path.abspath(root))
+    # FELHOM_WORKSPACE (R-1114): set by felhom-controller's runner to the MAIN clone's parent when <root> is a
+    # worktree outside git/ (its own parent holds other worktrees, not the sibling repos). The main clone of
+    # <root> itself is then skipped: a path deleted in the worktree must not resolve in the main clone.
+    parent = os.environ.get("FELHOM_WORKSPACE") or os.path.dirname(os.path.abspath(root))
+    own_main = _main_clone_of_worktree(root)
     try:
         entries = sorted(os.listdir(parent))
     except OSError as e:
@@ -129,7 +149,7 @@ def find_siblings(root):
     sibs = []
     for e in entries:
         p = os.path.join(parent, e)
-        if os.path.abspath(p) == os.path.abspath(root):
+        if os.path.abspath(p) == os.path.abspath(root) or (own_main and os.path.realpath(p) == own_main):
             continue
         if os.path.isdir(p) and os.path.exists(os.path.join(p, ".git")):
             sibs.append(p)