fix(extensions/git): platform-aware persist hint and stronger SPECIFY_INIT_DIR test

- Add a shared _persist_hint() helper in create_new_feature_branch.py
  and use it for both the JSON-mode stderr hint and the human-readable
  stdout hint, so there is a single place emitting the SPECIFY_FEATURE
  persistence guidance. On Windows (os.name == "nt") it prints
  PowerShell $env:VAR = "..." syntax; elsewhere it keeps the existing
  POSIX export VAR=... syntax (parity with the bash twin).
- Rework test_specify_init_dir_resolves_target_project so SPECIFY_INIT_DIR
  is the only thing that can produce the observed result: the script now
  runs from a separate host_proj (no existing specs, so script/cwd-based
  discovery would yield 001) while SPECIFY_INIT_DIR points at a different
  target_proj that already has an existing spec (007-existing, so the
  override must yield 008). The old version pointed SPECIFY_INIT_DIR at
  the same project the script was installed in, so it passed even if the
  env var were ignored.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: marcelsafin

extensions/git/scripts/python/create_new_feature_branch.py

@@ -68,6 +68,13 @@ def _err(message: str) -> None:
     print(message, file=sys.stderr)
 
 
+def _persist_hint(var_name: str, value: str) -> str:
+    """Shell-appropriate guidance for persisting an env var in the caller's shell."""
+    if os.name == "nt":
+        return f'$env:{var_name} = "{value}"'
+    return f"export {var_name}={shlex.quote(value)}"
+
+
 @dataclass
 class Args:
     json_mode: bool = False
@@ -598,7 +605,7 @@ def build_branch_name(feature_num: str, branch_suffix: str) -> str:
                 f"creation for {branch_name}"
             )
 
-        _err(f"# To persist: export SPECIFY_FEATURE={shlex.quote(branch_name)}")
+        _err(f"# To persist: {_persist_hint('SPECIFY_FEATURE', branch_name)}")
 
     if args.json_mode:
         payload: dict[str, object] = {
@@ -613,8 +620,8 @@ def build_branch_name(feature_num: str, branch_suffix: str) -> str:
         print(f"FEATURE_NUM: {feature_num}")
         if not args.dry_run:
             print(
-                "# To persist in your shell: export "
-                f"SPECIFY_FEATURE={shlex.quote(branch_name)}"
+                "# To persist in your shell: "
+                f"{_persist_hint('SPECIFY_FEATURE', branch_name)}"
             )
 
     return 0

tests/extensions/git/test_git_extension_python_parity.py

@@ -295,18 +295,26 @@ def test_empty_description_errors(self, tmp_path: Path):
         assert "cannot be empty or contain only whitespace" in p.stderr
 
     def test_specify_init_dir_resolves_target_project(self, tmp_path: Path):
-        _, py_proj = _twin_projects(tmp_path)
+        # The script is installed under host_proj, so script_file-based
+        # discovery (and cwd-based discovery, since we run from elsewhere)
+        # would resolve host_proj, not target_proj. host_proj has no specs
+        # (next number 001); target_proj already has 007-existing (next
+        # number 008). Only honoring SPECIFY_INIT_DIR produces 008, so this
+        # proves the env var -- not script location or cwd -- controls
+        # resolution.
+        host_proj = _setup_py_project(tmp_path / "host")
+        target_proj = _setup_py_project(tmp_path / "target")
+        (target_proj / "specs" / "007-existing").mkdir(parents=True)
         elsewhere = tmp_path / "elsewhere"
         elsewhere.mkdir()
-        # Run from outside the project so SPECIFY_INIT_DIR is what resolves it.
         p = _run_py(
-            "create-new-feature-branch", py_proj,
+            "create-new-feature-branch", host_proj,
             "--json", "--dry-run", "init dir feature",
-            env_extra={"SPECIFY_INIT_DIR": str(py_proj)},
+            env_extra={"SPECIFY_INIT_DIR": str(target_proj)},
             run_cwd=elsewhere,
         )
         assert p.returncode == 0
-        assert json.loads(p.stdout)["FEATURE_NUM"] == "001"
+        assert json.loads(p.stdout)["FEATURE_NUM"] == "008"
 
     def test_specify_init_dir_without_core_errors(self, tmp_path: Path):
         _, py_proj = _twin_projects(tmp_path)