Regenerate _text/ on commit, so the readable copy cannot drift

_text/ is derived from the PNG tEXt chunks and settings.json, and derived data
regenerated by hand goes stale silently -- which is worse than absent, because a
stale card reads as current. A pre-commit hook now regenerates and stages it.

Blocking, unlike the halogen repo's advisory doc-check: that one reports a
judgement call, this one rebuilds a file. Install with
`./st-export.py --install-hook`, since hooks are not versioned.

Verified by renaming a persona in the untracked settings.json and committing
WITHOUT running the export -- _text/personas.json updated itself and was
included. Test edit reverted.

st-export.py

@@ -115,7 +115,40 @@ def export_personas() -> int:
     return len(out.get("personas") or {})
 
 
+HOOK = """#!/usr/bin/env bash
+# Installed by st-export.py --install-hook.
+# Regenerates _text/ from the PNGs and settings.json, then stages it, so the
+# readable copy can never drift from the artifact SillyTavern actually loads.
+set -e
+cd "$(git rev-parse --show-toplevel)"
+python3 ./st-export.py >/dev/null
+git add -A _text
+"""
+
+
+def install_hook() -> int:
+    import subprocess
+    root = subprocess.run(["git", "rev-parse", "--git-dir"], capture_output=True,
+                          text=True).stdout.strip()
+    if not root:
+        print("not a git repository")
+        return 1
+    path = os.path.join(root, "hooks", "pre-commit")
+    os.makedirs(os.path.dirname(path), exist_ok=True)
+    open(path, "w").write(HOOK)
+    os.chmod(path, 0o755)
+    print(f"armed: {path}")
+    # ⚠ BLOCKING ON PURPOSE, unlike the halogen repo's advisory doc-check. That
+    # one reports a judgement call; this one regenerates a derived file. If it
+    # fails, _text/ would silently describe a card that no longer exists — and a
+    # stale derived copy is worse than no copy, because it reads as current.
+    print("⚠ runs on every commit; a failure blocks it, which is the point")
+    return 0
+
+
 def main() -> int:
+    if "--install-hook" in sys.argv:
+        return install_hook()
     os.makedirs(OUT, exist_ok=True)
     cards = export_cards()
     personas = export_personas()