fix: pin interpreters whose path contains a space

[Souptik96]

Fixes #2166

Root cause (a bit narrower than the report)

The empty _PINNED comes from the install-time allowlist in _pinned_python(), not from the
uv-tool layout. It accepted [a-zA-Z0-9/_.@:\-] — no space — and rejects the path
wholesale on any non-matching character, so sys.executable under a profile whose name
contains a space records nothing.

Verified on Windows against v8 (0.9.26), stubbing sys.executable:

PINNED   'C:\Users\souptic\AppData\Roaming\uv\tools\graphifyy\Scripts\python.exe'  -> pinned
EMPTY!!  'C:\Users\First Last\AppData\Roaming\uv\tools\graphifyy\Scripts\python.exe' -> ''
PINNED   '/home/u/.local/share/uv/tools/graphifyy/bin/python'                      -> pinned
EMPTY!!  'C:\Users\evil\py.exe; rm -rf /'                                          -> ''   (correct)
EMPTY!!  "C:\Users\ev'il\python.exe"                                               -> ''   (correct)

So a space-free Windows uv path pins fine — which is why this reads as uv-layout-specific.
What actually breaks it is a space anywhere in the interpreter path: C:\Users\First Last\...,
or the equally common C:\Program Files\Python312\python.exe. Once _PINNED is empty, probe 1
is skipped and (per the report's own table) probes 2–4 can't recover on a uv install, so every
commit warns and no-ops.

That also means the issue's other suggestion — writing graphify-out/.graphify_python at hook
install time — isn't needed to fix this; probe 1 works once it's actually populated. I left it
out to keep the change minimal.

Fix

• _pinned_python(): allow a plain space. This is safe because every consumer already
  quotes the value — the hook embeds _PINNED='<path>' (single-quoted) and dereferences
  "$_PINNED", so a space can neither split a word nor start a command. $, backtick, ;,
  ' and " remain rejected, so the injection guard is unchanged.
• _register_merge_driver(): quote the interpreter. It interpolated the path unquoted
  into merge.graphify.driver, which git runs through a shell — a spaced path would split into
  two words and the driver would never run. Now double-quoted, which is safe precisely because
  the allowlist keeps $/backticks out.

I deliberately did not convert the pinned path to forward slashes (the issue suggests
sys.executable.replace("\\", "/")). Probe 1 quotes "$_PINNED" so backslashes are fine there,
#2126 already established that a pinned C:\... path is the expected shape, and rewriting it
would churn the existing test_install_embeds_pinned_interpreter assertion for no gain.

Testing

New tests in tests/test_hooks.py:

$ uv run pytest tests/test_hooks.py -q -k "pinned_python_accepts_paths_containing_spaces or pinned_python_still_rejects or merge_driver_quotes_interpreter or install_pins_interpreter_path_with_spaces"
11 passed, 67 deselected, 1 warning in 1.29s

Confirmed they actually catch the bug — same tests with graphify/hooks.py reverted to v8:

FAILED tests/test_hooks.py::test_pinned_python_accepts_paths_containing_spaces[C:\Users\First Last\...\python.exe]
FAILED tests/test_hooks.py::test_pinned_python_accepts_paths_containing_spaces[C:\Program Files\Python312\python.exe]
FAILED tests/test_hooks.py::test_pinned_python_accepts_paths_containing_spaces[/home/first last/...]
FAILED tests/test_hooks.py::test_merge_driver_quotes_interpreter_with_spaces - interpreter not quoted in merge driver: 'graphify merge-dri...'
FAILED tests/test_hooks.py::test_install_pins_interpreter_path_with_spaces - post-commit did not pin the spaced interpreter
5 failed, 73 deselected

Affected files plus both pre-commit gates:

$ uv run pytest tests/test_hooks.py tests/test_hook_strict.py tests/test_install.py -q
11 failed, 172 passed, 1 warning in 19.40s

$ uv run ruff check --config pyproject.toml graphify/hooks.py tests/test_hooks.py
All checks passed!

$ uv run python -m tools.skillgen --check
check OK: 134 artifact(s) match committed output and expected/.

Those 11 failures are pre-existing on Windows, not from this PR. Baseline uv run pytest tests/ -q on unmodified v8 (66d8110) is 45 failed, 3578 passed, 15 skipped, of which
exactly 11 live in these three files (test_hooks.py 8, test_hook_strict.py 1,
test_install.py 2) — the same 11, and most of the test_hooks.py ones are the
bash-dependent allowlist tests plus WinError cases. I have no Linux/macOS box to confirm
they're green there, so please read that number in that light.