feat(extensions): port git extension scripts to Python (#3400)

* feat(extensions): port git extension scripts to Python

Ports git-common, initialize-repo, auto-commit, and
create-new-feature-branch to extensions/git/scripts/python/,
mirroring the bash/PowerShell twins. Parity tests run each bash
script and its Python twin in identical projects and compare
output, exit codes, and resulting git state.

Fixes #3282

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

* fix: match bash error message for whitespace-only descriptions

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

* Handle unreadable git-config.yml and assert stderr parity

An unreadable config file raised OSError with a full traceback from
_parse_auto_commit_config. Treat it like a missing config: auto-commit
stays disabled. Covered by a chmod-000 test (skipped on non-POSIX and
as root).

_assert_parity now also compares stderr so warning or usage-text
regressions between the bash and Python twins fail the suite. All
existing parity tests pass with the stricter assertion.

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

* fix(extensions/git): pass script path to core.get_repo_root for cwd-outside-repo callers

Without script_file, core.get_repo_root() falls back to Path.cwd() when
SPECIFY_INIT_DIR is unset and no .specify root is found upward — the
bash twin instead falls back to the script's install location
(.specify/scripts/...). Pass script_file so both twins resolve the
same repo_root; TypeError fallback keeps older cores working.

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

* test: exercise SPECIFY_INIT_DIR from outside the project

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

* fix(extensions/git): handle UnicodeDecodeError and USER/USERNAME fallback

- Catch (OSError, UnicodeDecodeError) when reading git-config.yml in
  create_new_feature_branch.py, initialize_repo.py, and auto_commit.py
  so invalid UTF-8 config falls back to defaults instead of crashing
  with a traceback.
- Fall back to USERNAME (then "unknown") when USER is unset when
  deriving the branch author token, matching the PowerShell twin's
  Windows-friendly fallback chain.

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

* 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>

* fix(extensions): tolerate missing Git executable

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

* fix(extensions): quote PowerShell persist hint

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

* fix(git): match bash persist hint escaping

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

* fix(git): ignore unterminated config record

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

* test(git): handle Windows persist hint parity

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

* fix(init): install Python shared scripts

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

* test(git): normalize Windows persistence hints

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

---------

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

extensions/git/scripts/python/git_common.py

@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+"""Git-specific common helpers for the git extension.
+
+Python port of ``git-common.sh`` / ``git-common.ps1`` — contains only
+git-specific branch validation and detection logic.
+"""
+
+from __future__ import annotations
+
+import re
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+
+
+def has_git(repo_root: Path | None = None) -> bool:
+    """Check if we have git available at the repo root."""
+    root = Path(repo_root) if repo_root is not None else Path.cwd()
+    git_marker = root / ".git"
+    if not (git_marker.is_dir() or git_marker.is_file()):
+        return False
+    if shutil.which("git") is None:
+        return False
+    result = subprocess.run(
+        ["git", "-C", str(root), "rev-parse", "--is-inside-work-tree"],
+        capture_output=True,
+        text=True,
+    )
+    return result.returncode == 0
+
+
+def effective_branch_name(raw: str) -> str:
+    """Strip a single optional path segment (e.g. gitflow "feat/004-name" -> "004-name").
+
+    Only when the full name is exactly two slash-free segments; otherwise
+    returns the raw name.
+    """
+    match = re.fullmatch(r"([^/]+)/([^/]+)", raw)
+    if match:
+        return match.group(2)
+    return raw
+
+
+def check_feature_branch(raw: str, has_git_repo: bool) -> bool:
+    """Validate that a branch name matches the expected feature branch pattern.
+
+    Accepts sequential (###-* with >=3 digits) or timestamp (YYYYMMDD-HHMMSS-*)
+    formats, either at the start of the branch or after path-style namespace
+    prefixes. Logic aligned with the bash/PowerShell twins.
+    """
+    if not has_git_repo:
+        print(
+            "[specify] Warning: Git repository not detected; skipped branch validation",
+            file=sys.stderr,
+        )
+        return True
+
+    branch = effective_branch_name(raw)
+    feature_segment = branch.rsplit("/", 1)[-1]
+
+    # Accept sequential prefix (3+ digits) but exclude malformed timestamps:
+    # 7-or-8 digit date + 6-digit time with no trailing slug.
+    is_sequential = bool(
+        re.match(r"^[0-9]{3,}-", feature_segment)
+        and not re.match(r"^[0-9]{7}-[0-9]{6}-", feature_segment)
+        and not re.fullmatch(r"[0-9]{7,8}-[0-9]{6}", feature_segment)
+    )
+    is_timestamp = bool(re.match(r"^[0-9]{8}-[0-9]{6}-", feature_segment))
+
+    if not is_sequential and not is_timestamp:
+        print(f"ERROR: Not on a feature branch. Current branch: {raw}", file=sys.stderr)
+        print(
+            "Feature branches should be named like: 001-feature-name, "
+            "1234-feature-name, 20260319-143022-feature-name, or "
+            "<prefix>/001-feature-name",
+            file=sys.stderr,
+        )
+        return False
+
+    return True