Every 'hermes update' now runs a full backup of ~/.hermes/ first, so
users can always roll back to the exact state they had before the
update if anything goes wrong (corrupted sessions.db, broken skills,
config migrations that don't round-trip, etc.).
Changes:
- hermes_cli/backup.py: new create_pre_update_backup() helper. Writes
to <HERMES_HOME>/backups/pre-update-<stamp>.zip using the same
exclusion rules and SQLite safe-copy as 'hermes backup'. Auto-rotates
(keep last N, pre-update-*.zip only — hand-dropped zips in backups/
are untouched). Adds 'backups' to _EXCLUDED_DIRS so subsequent backups
don't nest prior ones.
- hermes_cli/main.py: _run_pre_update_backup() wired into
_cmd_update_impl before any git operation. Prints save path, restore
command, and how to disable. Swallows failures so a broken backup
never blocks the update itself. New --no-backup flag on 'hermes
update' for one-off override.
- hermes_cli/config.py: new 'updates' section in DEFAULT_CONFIG with
pre_update_backup (default true) and backup_keep (default 5).
Auto-surfaces in the dashboard config UI.
- tests/hermes_cli/test_backup.py: +11 tests covering backup location,
content parity with 'hermes backup', no-recursion, rotation, manual
file preservation, config gate, --no-backup flag, flag-wins-over-config.
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