Tests: add coverage for _migrate_config — malformed input, already-migrated config, and unknown key preservation

[ChaseFlorell]

Problem

_migrate_config is called at the start of --start and --update. It deep-merges CONFIG_DEFAULTS into the existing config.toml and writes back any missing keys. Bugs here affect every user on every run. Key edge cases are untested:

• Malformed config.toml: currently caught and printed, but no test verifies this
• Already-migrated config: calling _migrate_config on a fully up-to-date config should not write the file (to avoid unnecessary disk I/O and timestamp changes)
• Unknown user keys: keys that exist in config.toml but not in CONFIG_DEFAULTS (e.g. a user-added comment or a key from an older version) should be preserved, not dropped

Acceptance Criteria

• Malformed config.toml produces a human-readable error message and does not crash with a raw traceback
• _migrate_config on an already-complete config does not call save_config (no disk write)
• User keys not in CONFIG_DEFAULTS survive a _migrate_config round-trip

Proposed Tests

• test_migrate_config_handles_malformed_toml — write invalid TOML; assert error printed, no crash
• test_migrate_config_no_write_when_already_complete — mock save_config; assert it is not called when config already has all keys
• test_migrate_config_preserves_unknown_keys — add a non-default key to config; assert it is present after migration
• test_migrate_config_adds_missing_keys — config missing a key from CONFIG_DEFAULTS; assert key is added after migration