refactor: consolidate SKILL.md version regex into lib/skill_meta.py

The same `^version:\s*"([^"]+)"\s*$` regex (or a slight variant) was
duplicated across three files: render.py inline, test_plugin_contract.py
local helper, test_version_consistency.py local helper. A future change
to the SKILL.md frontmatter version format would have needed to update
three places without any compile-time pressure to keep them in sync.

New skills/last30days/scripts/lib/skill_meta.py provides:
- `_VERSION_RE` private compiled pattern (accepts double-quoted,
  single-quoted, or unquoted YAML version scalars per the widening
  landed in 997708a)
- `read_skill_version(skill_md_path: Path) -> str | None` helper that
  catches OSError + UnicodeDecodeError and returns None on miss

Callers updated:
- render.py::_skill_version now calls skill_meta.read_skill_version
  inside the SKILL.md fallback loop, returning `read_skill_version(...) or "?"`.
  Semantically equivalent to the old break-after-first-SKILL.md logic.
- test_plugin_contract.py and test_version_consistency.py import the
  helper instead of defining the regex inline. Both files use the
  established sys.path.insert pattern.

Added tests/test_skill_meta.py with 6 direct unit tests covering the
helper's full contract: missing file, undecodable bytes, no-version-line,
and all three quoting styles (double, single, unquoted). Previously the
helper was only exercised transitively through render._skill_version().

Added test_skill_md_uses_double_quoted_version to
test_version_consistency.py — the old per-test regex incidentally
asserted "this repo's SKILL.md uses double-quotes" by being strict;
the shared helper accepts all three styles, so the assertion is now
explicit instead of implicit.

Code-reviewed by ce-code-review (8 reviewers); safe_auto fixes applied
inline (rename to _VERSION_RE, group or-chain instead of generator,
docstring tightened, dropped unnecessary `from __future__ import
annotations`, tightened signature to Path-only).

Conftest.py refactor for the sys.path.insert duplication across ~20 test
files filed as issue #411 — out of scope for this PR (touches many
files, separate concern).

Test results: 23 passed in the affected test set (16 prior + 6 new
test_skill_meta tests + 1 new double-quote assertion). Full suite shows
same 13 pre-existing failures as main; zero new failures.

tests/test_plugin_contract.py

@@ -1,5 +1,5 @@
 import json
-import re
+import sys
 import tomllib
 import unittest
 from pathlib import Path
@@ -8,17 +8,19 @@ from pathlib import Path
 ROOT = Path(__file__).resolve().parents[1]
 SKILL_ROOT = ROOT / "skills" / "last30days"
 
+sys.path.insert(0, str(SKILL_ROOT / "scripts"))
+from lib.skill_meta import read_skill_version  # noqa: E402
+
 
 def _json(path: Path) -> dict:
     return json.loads(path.read_text(encoding="utf-8"))
 
 
 def _skill_version() -> str:
-    text = (SKILL_ROOT / "SKILL.md").read_text(encoding="utf-8")
-    match = re.search(r'^version:\s*"([^"]+)"\s*$', text, re.MULTILINE)
-    if not match:
+    version = read_skill_version(SKILL_ROOT / "SKILL.md")
+    if not version:
         raise AssertionError("SKILL.md version frontmatter not found")
-    return match.group(1)
+    return version
 
 
 class TestPluginContract(unittest.TestCase):