fix(update): adopting a checkout git cannot identify writes the birth certificate again

write_source_stamp now returns None for a tree git cannot identify, instead
of raising. Blessed-checkout adoption still caught RuntimeError for its
minimal-stamp fallback, so it wrote nothing, reported the root as adopted,
and a read-only tree no longer reported itself unwritable. Branch on the
None the function returns now.

hermes_cli/post_update.py

@@ -206,18 +206,18 @@ def step_adopt_blessed_checkout(project_root: Path | None = None) -> dict:
     if not is_blessed:
         return {"ok": True, "skipped": "not-a-blessed-root"}
 
-    try:
-        from hermes_cli.source_stamp import write_source_stamp
+    from hermes_cli.source_stamp import write_source_stamp
 
+    try:
         # Full checkout identity when git can answer; the birth-certificate
         # minimum below only when it cannot (or the tree is read-only).
-        write_source_stamp(root)
+        identified = write_source_stamp(root)
     except OSError as exc:
         # A read-only tree (nix-like layouts without their own stamp)
         # must not crash the boot — it just stays unadopted.
         logger.debug("blessed-checkout adoption skipped (unwritable): %s", exc)
         return {"ok": True, "skipped": f"unwritable: {exc}"}
-    except RuntimeError:
+    if identified is None:
         # No derivable git identity: write the minimal birth certificate.
         stamp = {
             "schemaVersion": 2,