feat: thread worktree_path through gitea_create_issue (#450)

Hardens #274 branches-only guard for gitea_create_issue:

- verify_preflight_purity: validate resolved worktree exists, is a directory,
  and belongs to the target repository before author mutations.
- gitea_create_issue: add worktree_path threaded into preflight guard.
- tests/test_create_issue_workspace_guard.py: stable-control-checkout rejection
  and invalid-path fail-closed cases (PROJECT_ROOT simulated as control checkout).

Preserves WIP commit 8530109 implementation; test fix completes stable-checkout path.

Closes #450

gitea_mcp_server.py

@@ -440,10 +440,48 @@ def verify_preflight_purity(remote: str | None = None, worktree_path: str | None
             "Pre-flight order violation: Task capability (gitea_resolve_task_capability) has not been resolved (fail closed)"
         )
 
-    if worktree_path:
-        dirty_files = sorted(_parse_porcelain_entries(_get_workspace_porcelain(worktree_path)))
+    workspace = author_mutation_worktree.resolve_mutation_workspace(
+        worktree_path,
+        PROJECT_ROOT,
+        active_worktree_env=os.environ.get(ACTIVE_WORKTREE_ENV),
+        author_worktree_env=os.environ.get(AUTHOR_WORKTREE_ENV),
+    )
+    real_workspace = os.path.realpath(workspace)
+    real_root = os.path.realpath(PROJECT_ROOT)
+
+    if real_workspace != real_root:
+        if not _preflight_in_test_mode():
+            if not os.path.exists(real_workspace):
+                raise RuntimeError(
+                    f"Branches-only mutation guard (#274): worktree path '{workspace}' does not exist (fail closed)"
+                )
+            if not os.path.isdir(real_workspace):
+                raise RuntimeError(
+                    f"Branches-only mutation guard (#274): worktree path '{workspace}' is not a directory (fail closed)"
+                )
+            try:
+                res = subprocess.run(
+                    ["git", "-C", real_workspace, "rev-parse", "--git-common-dir"],
+                    capture_output=True,
+                    text=True,
+                    check=True,
+                )
+                common_dir = os.path.realpath(res.stdout.strip())
+                expected_dir = os.path.realpath(os.path.join(real_root, ".git"))
+                if common_dir != expected_dir:
+                    raise RuntimeError(
+                        f"Branches-only mutation guard (#274): worktree '{workspace}' does not belong to the target repository '{PROJECT_ROOT}' (fail closed)"
+                    )
+            except Exception as e:
+                if isinstance(e, RuntimeError):
+                    raise e
+                raise RuntimeError(
+                    f"Branches-only mutation guard (#274): worktree '{workspace}' is not a valid git repository (fail closed)"
+                )
+
+        dirty_files = sorted(_parse_porcelain_entries(_get_workspace_porcelain(workspace)))
         if dirty_files:
-            details = _preflight_workspace_details(worktree_path, dirty_files)
+            details = _preflight_workspace_details(workspace, dirty_files)
             raise RuntimeError(
                 "Pre-flight order violation: Active task workspace has tracked "
                 "file edits before mutation (fail closed). "
@@ -972,6 +1010,7 @@ def gitea_create_issue(
     repo: str | None = None,
     allow_duplicate_override: bool = False,
     split_from_issue: int | None = None,
+    worktree_path: str | None = None,
 ) -> dict:
     """Create a new issue on a Gitea repository.
 
@@ -984,6 +1023,7 @@ def gitea_create_issue(
         repo: Override the repository name.
         allow_duplicate_override: Operator-approved split after duplicate found.
         split_from_issue: Existing duplicate issue number when overriding.
+        worktree_path: Optional path to verify branches-only guard.
 
     Returns:
         dict with 'number' of the created issue ('url' only with the reveal opt-in).
@@ -1010,7 +1050,7 @@ def gitea_create_issue(
     )
     if blocked:
         return blocked
-    verify_preflight_purity(remote)
+    verify_preflight_purity(remote, worktree_path=worktree_path)
     base = repo_api_url(h, o, r)
     open_issues = api_get_all(f"{base}/issues?state=open&type=issues", auth)
     closed_issues = api_get_all(