fix(backup): floor pre-update backup_keep to 1 so the new backup survives

`updates.backup_keep: 0` (or any negative value) wiped the freshly-
created pre-update zip:

  _prune_pre_update_backups(backup_dir, keep=0):
      backups = sorted(..., reverse=True)   # newest first, includes
                                            # the zip we just wrote
      for p in backups[0:]:                 # = all of them
          p.unlink()

The wrapper in `main.py` then printed `Saved: <path>` for a file that
no longer existed (the size lookup is wrapped in `try/except OSError`
which silently degrades to "0 B"), leaving operators believing they had
a recovery point when they had none.

This is a real footgun because some config systems treat 0 as "keep
unlimited"; here it does the opposite — every backup is destroyed
right after creation.

Fix: clamp `keep` to a minimum of 1 inside `_prune_pre_update_backups`
since that helper is only invoked immediately after a fresh backup
is written.  Operators who genuinely want no backups should set
`updates.pre_update_backup: false` (which gates creation entirely)
rather than relying on `backup_keep: 0`.

Also extends the `backup_keep` config docstring to spell out the floor
and point at `pre_update_backup: false` as the off-switch.

## Tests

Three regression tests added in `TestPreUpdateBackup`:

  - `test_keep_zero_does_not_delete_freshly_created_backup` —
    asserts the file persists after `keep=0`
  - `test_keep_negative_does_not_delete_freshly_created_backup` —
    same for negative values
  - `test_keep_zero_still_prunes_older_backups` — proves the floor
    only protects the new backup; older ones are still rotated out

Verified the new tests fail on origin/main (without the floor) and
pass with it; full `tests/hermes_cli/test_backup.py` suite green
(84 tests).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hermes_cli/backup.py

@@ -793,9 +793,17 @@ def _prune_pre_update_backups(backup_dir: Path, keep: int) -> int:
     Returns the number of files deleted.  Only touches files matching
     ``pre-update-*.zip`` so hand-made zips dropped in the same directory
     are never touched.
+
+    ``keep`` is floored to 1 because this helper is only called immediately
+    after a fresh backup is written: deleting that backup right after the
+    user paid the disk/CPU cost to create it would leave them worse off
+    than no backup at all (and the wrapper in ``main.py`` would still print
+    a misleading ``Saved: <path>`` line for a file that no longer exists).
+    Operators who genuinely don't want a backup should set
+    ``updates.pre_update_backup: false`` in config — that gates creation.
     """
-    if keep < 0:
-        keep = 0
+    if keep < 1:
+        keep = 1
     if not backup_dir.exists():
         return 0
 

hermes_cli/config.py

@@ -1286,7 +1286,10 @@ DEFAULT_CONFIG = {
         # for a single update run.
         "pre_update_backup": False,
         # How many pre-update backup zips to retain.  Older ones are pruned
-        # automatically after each successful backup.
+        # automatically after each successful backup.  Values below 1 are
+        # floored to 1 — the backup just created is always preserved.  To
+        # disable backups entirely, set ``pre_update_backup: false`` above
+        # rather than ``backup_keep: 0``.
         "backup_keep": 5,
     },