feat: enforce branches-only author mutation worktree guard (Closes #274)

Add author_mutation_worktree assessment and integrate it into MCP
preflight so author mutations fail closed outside branches/ session
worktrees, with tests for control-checkout and branches-path cases.

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

gitea_mcp_server.py

@@ -397,6 +397,28 @@ def record_preflight_check(type_name: str, resolved_role: str | None = None):
             _preflight_resolved_role = resolved_role
 
 
+def _enforce_branches_only_author_mutation(worktree_path: str | None = None) -> None:
+    """#274: author mutations must run from a branches/ session worktree."""
+    if _preflight_resolved_role == "reviewer":
+        return
+    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),
+    )
+    git_state = issue_lock_worktree.read_worktree_git_state(workspace)
+    assessment = author_mutation_worktree.assess_author_mutation_worktree(
+        workspace_path=workspace,
+        project_root=PROJECT_ROOT,
+        current_branch=git_state.get("current_branch"),
+    )
+    if assessment["block"]:
+        raise RuntimeError(
+            author_mutation_worktree.format_author_mutation_worktree_error(assessment)
+        )
+
+
 def verify_preflight_purity(remote: str | None = None, worktree_path: str | None = None):
     """Verify that identity and capability were verified prior to session edits."""
     global _preflight_reviewer_violation_files
@@ -426,32 +448,33 @@ def verify_preflight_purity(remote: str | None = None, worktree_path: str | None
                 "file edits before mutation (fail closed). "
                 f"{_format_preflight_workspace_details(details)}"
             )
-        return
-
-    if _preflight_whoami_violation:
-        raise RuntimeError(
-            "Pre-flight order violation: Workspace file edits occurred before "
-            f"gitea_whoami verification (fail closed). Offending files: "
-            f"{_format_preflight_files(_preflight_whoami_violation_files)}"
-        )
-    if _preflight_capability_violation:
-        raise RuntimeError(
-            "Pre-flight order violation: Workspace file edits occurred before "
-            f"gitea_resolve_task_capability verification (fail closed). Offending files: "
-            f"{_format_preflight_files(_preflight_capability_violation_files)}"
-        )
-
-    if _preflight_resolved_role == "reviewer":
-        current = _get_workspace_porcelain()
-        baseline = _preflight_capability_baseline_porcelain or ""
-        reviewer_delta = _new_tracked_changes_since(baseline, current)
-        _preflight_reviewer_violation_files = reviewer_delta
-        if reviewer_delta:
+    else:
+        if _preflight_whoami_violation:
             raise RuntimeError(
-                "Reviewer role violation: Reviewer profile is forbidden from modifying "
-                "tracked workspace files (fail closed). Offending files: "
-                f"{_format_preflight_files(reviewer_delta)}"
+                "Pre-flight order violation: Workspace file edits occurred before "
+                f"gitea_whoami verification (fail closed). Offending files: "
+                f"{_format_preflight_files(_preflight_whoami_violation_files)}"
             )
+        if _preflight_capability_violation:
+            raise RuntimeError(
+                "Pre-flight order violation: Workspace file edits occurred before "
+                f"gitea_resolve_task_capability verification (fail closed). Offending files: "
+                f"{_format_preflight_files(_preflight_capability_violation_files)}"
+            )
+
+        if _preflight_resolved_role == "reviewer":
+            current = _get_workspace_porcelain()
+            baseline = _preflight_capability_baseline_porcelain or ""
+            reviewer_delta = _new_tracked_changes_since(baseline, current)
+            _preflight_reviewer_violation_files = reviewer_delta
+            if reviewer_delta:
+                raise RuntimeError(
+                    "Reviewer role violation: Reviewer profile is forbidden from modifying "
+                    "tracked workspace files (fail closed). Offending files: "
+                    f"{_format_preflight_files(reviewer_delta)}"
+                )
+
+    _enforce_branches_only_author_mutation(worktree_path)
 
 from mcp.server.fastmcp import FastMCP  # noqa: E402
 
@@ -476,6 +499,7 @@ import task_capability_map  # noqa: E402
 import review_proofs  # noqa: E402
 import agent_temp_artifacts
 import issue_lock_worktree  # noqa: E402
+import author_mutation_worktree  # noqa: E402
 import merged_cleanup_reconcile  # noqa: E402