fix: detect correct Python interpreter for pipx installs in git hooks

* fix: git hooks fail when graphify is installed via pipx

When installed via pipx, the graphify module is only available in
pipx's isolated venv, not the system python3. The git hooks
(post-commit, post-checkout) hardcoded `python3` which cannot import
graphify in this case.

Detect the correct Python interpreter from the graphify binary's
shebang line, matching the approach already used in skill.md Step 1.
Falls back to python3 for system installs.

* fix: handle env-style shebangs and improve interpreter detection

- Use POSIX `command -v` instead of non-standard `which`
- Parse `#!/usr/bin/env python3` shebangs correctly (previous
  `tr -d ' '` would produce `/usr/bin/envpython3`)
- Add import validation fallback to python3 if resolved interpreter
  cannot import graphify

graphify/hooks.py

@@ -8,6 +8,23 @@ _HOOK_MARKER_END = "# graphify-hook-end"
 _CHECKOUT_MARKER = "# graphify-checkout-hook-start"
 _CHECKOUT_MARKER_END = "# graphify-checkout-hook-end"
 
+_PYTHON_DETECT = """\
+# Detect the correct Python interpreter (handles pipx, venv, system installs)
+GRAPHIFY_BIN=$(command -v graphify 2>/dev/null)
+if [ -n "$GRAPHIFY_BIN" ]; then
+    _SHEBANG=$(head -1 "$GRAPHIFY_BIN" | sed 's/^#![[:space:]]*//')
+    case "$_SHEBANG" in
+        */env\\ *) GRAPHIFY_PYTHON="${_SHEBANG#*/env }" ;;
+        *)         GRAPHIFY_PYTHON="$_SHEBANG" ;;
+    esac
+    if ! "$GRAPHIFY_PYTHON" -c "import graphify" 2>/dev/null; then
+        GRAPHIFY_PYTHON="python3"
+    fi
+else
+    GRAPHIFY_PYTHON="python3"
+fi
+"""
+
 _HOOK_SCRIPT = """\
 # graphify-hook-start
 # Auto-rebuilds the knowledge graph after each commit (code files only, no LLM needed).
@@ -18,8 +35,9 @@ if [ -z "$CHANGED" ]; then
     exit 0
 fi
 
+""" + _PYTHON_DETECT + """
 export GRAPHIFY_CHANGED="$CHANGED"
-python3 -c "
+$GRAPHIFY_PYTHON -c "
 import os, sys
 from pathlib import Path
 
@@ -67,8 +85,9 @@ if [ ! -d "graphify-out" ]; then
     exit 0
 fi
 
+""" + _PYTHON_DETECT + """
 echo "[graphify] Branch switched - rebuilding knowledge graph (code files)..."
-python3 -c "
+$GRAPHIFY_PYTHON -c "
 from graphify.watch import _rebuild_code
 from pathlib import Path
 import sys