harden hook interpreter detection against injection and unquoted exec

Per adversarial review of the #1127 fix:

- Apply the filesystem-path allowlist to sys.executable before embedding it in
  the generated hook script. Paths with metacharacters outside [a-zA-Z0-9/_.@:\-]
  (spaces, dollar signs, backticks, semicolons) are replaced with an empty string
  so the pinned probe is safely skipped rather than injecting shell commands that
  execute on every git commit.
- Change _PINNED='...' to single-quote assignment so no shell expansion can occur
  even if a character slipped through the allowlist (belt and suspenders).
- Quote $GRAPHIFY_PYTHON in both nohup exec lines so paths with spaces work
  (common in Windows C:\Program Files\... installs).
- Update test to assert the sanitized value, not raw sys.executable, so the test
  stays correct after any future sanitization changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_hooks.py

@@ -172,14 +172,16 @@ def test_install_embeds_pinned_interpreter(tmp_path):
     fallbacks cannot import graphify (wrong venv), and the hook silently exits 0.
     Pinning sys.executable at install time makes the hook work regardless of PATH.
     """
-    import sys
+    import re, sys
     repo = _make_git_repo(tmp_path)
     install(repo)
     commit_hook = (repo / ".git" / "hooks" / "post-commit").read_text()
     checkout_hook = (repo / ".git" / "hooks" / "post-checkout").read_text()
-    # The pinned interpreter path must appear in both generated scripts.
-    assert sys.executable in commit_hook, "pinned sys.executable missing from post-commit"
-    assert sys.executable in checkout_hook, "pinned sys.executable missing from post-checkout"
+    # Compute the sanitized value the same way install() does.
+    expected = sys.executable if not re.search(r"[^a-zA-Z0-9/_.@:\\-]", sys.executable) else ""
+    if expected:
+        assert expected in commit_hook, "sanitized sys.executable missing from post-commit"
+        assert expected in checkout_hook, "sanitized sys.executable missing from post-checkout"
     # The placeholder must be fully substituted -- no __PINNED_PYTHON__ left.
     assert "__PINNED_PYTHON__" not in commit_hook, "placeholder not substituted in post-commit"
     assert "__PINNED_PYTHON__" not in checkout_hook, "placeholder not substituted in post-checkout"