fix: the gate missed a group id quoted half-way

A journal line pasted into a commit message stopped at
"group.<first few characters>...", which matched neither the full id nor the
decoded one, so the check passed and the id shipped. Eight base64 characters
now count as that group: 48 bits, unique enough that a coincidence does not
matter, short enough to catch a truncated quotation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017w1HZ3cZ7neL3rFdQ6VMBA

scripts/personal_data_sources.py

@@ -28,6 +28,11 @@ REPO: Final = Path(__file__).resolve().parent.parent
 # popes, which is a cost worth paying to catch a real one.
 MIN_NAME: Final = 3
 
+# How much of a group id has to appear before it counts as that group. Eight
+# base64 characters are 48 bits: unique enough that a coincidence is not
+# worth worrying about, short enough to catch a truncated quotation.
+PREFIX: Final = 8
+
 
 def _pack_names(directory: Path) -> set[str]:
     """Every trigger word in a photo pack manifest -- i.e. every name."""
@@ -104,6 +109,10 @@ def _database_groups(path: Path) -> set[str]:
     inner: set[str] = set()
     for group in found:
         payload = group.removeprefix("group.")
+        # The leading run as well as the whole id: a log line quoted into a
+        # commit message says "group.<first few chars>..." and stops, and
+        # matching only the full id let exactly that through once.
+        inner.add(payload[:PREFIX])
         try:
             inner.add(base64.b64decode(payload, validate=True).decode("utf-8"))
         except ValueError, UnicodeDecodeError: