feat(backup): exclude checkpoints/ from backups (#16572)

Session-local trajectory cache — keyed by session hash, regenerated
per-session, won't port to another machine anyway. On a large install
this was multiple GB of pure noise in every zip.

Also adds a regression test for the pre-existing backups/ exclusion
so the two machine-local dirs share coverage.

hermes_cli/backup.py

@@ -37,6 +37,8 @@ _EXCLUDED_DIRS = {
     ".git",             # nested git dirs (profiles shouldn't have these, but safety)
     "node_modules",     # js deps if website/ somehow leaks in
     "backups",          # prior auto-backups — don't nest backups exponentially
+    "checkpoints",      # session-local trajectory caches — regenerated per-session,
+                        # session-hash-keyed so they don't port to another machine anyway
 }
 
 # File-name suffixes to skip