perf(hooks): eliminate multi-second foreground stalls before the detached launch

[TNRealtorApp]

Problem

On Windows (and any machine with a cold import cache or AV-scanned site-packages), git commit stalls for 10-40+ seconds inside the graphify post-commit hook before the #1161 detached launcher even starts:

1. Interpreter probes import the full package. Each probe runs the equivalent of python -c 'import graphify', which executes the whole package import — measured 13s per probe (cold) on a Windows 11 dev box — and up to four probes can run synchronously per commit.
2. The shebang probe reads a binary. Git for Windows' command -v graphify can return the launcher path without its .exe suffix, so the *.exe) guard misses and head -1 reads a PE binary: the shell prints warning: command substitution: ignored null byte in input on every commit, and the garbage 'shebang' always fails the allowlist and falls through to the slowest fallbacks (including the Microsoft Store python3 alias).
3. Redundant git rev-parse. git already exports GIT_DIR to hooks; the unconditional re-derive costs another ~1.3s on machines where every git exec is AV-scanned.

Fix

• Probes use importlib.util.find_spec('graphify') — locates the package without executing it, so each probe costs interpreter startup only. A broken install under the selected interpreter still fails loudly in the rebuild log (same as today). Probe order and fallback semantics are unchanged.
• The Windows pip layout is resolved directly: Scripts/graphify(.exe) → sibling ..\python.exe (or .\python.exe for venv Scripts dirs) — no binary parsing, works with or without the .exe suffix.
• The remaining POSIX shebang read strips NUL bytes first (head -c 256 | tr -d '\000' | head -n 1).
• GIT_DIR=${GIT_DIR:-$(git rev-parse --git-dir 2>/dev/null)} in both hooks (rev-parse remains the manual-invocation fallback).

Measured (Windows 11, Python 3.13, real repo)

	before	after
hook foreground (cold)	26s+ (2 x 13s import probes)	~1.4s (one find_spec probe)
null-byte warnings	every commit	none

Tests

tests/test_hooks.py: 47 passed (4 new — find_spec probes, NUL-safe shebang read, sibling python.exe resolution, GIT_DIR reuse; plus the _launcher_payload docstring updated to match).

🤖 Generated with Claude Code

[safishamsi]

Merged into v8 as 1256d65 (your authorship). Verified: probes now use find_spec (no full package import), NUL-safe shebang read, Windows sibling-python resolution, and GIT_DIR reuse — 4 new tests + full suite 2800 green. The find_spec/detached tradeoff is sound (foreground picks the interpreter; the detached rebuild still logs a broken install). Genuinely nice perf work for Windows/cold-cache hooks. Ships next release.