Add `partio configure` command for post-setup configuration changes

[jcleira]

Summary

Once a user has run partio enable, changing their agent or strategy requires partio disable && partio enable. There is no dedicated command for updating configuration after initial setup. This creates unnecessary friction for users who want to change settings without reinstalling hooks.

Proposed Change

Add a partio configure command with flags to update individual settings in .partio/settings.json:

partio configure --agent <agent-name>
partio configure --strategy <strategy-name>
partio configure              # prints current config

Inspired by entireio/cli#851 (entire configure to manage agents).

Acceptance Criteria

• partio configure command exists and is registered in the root command
• partio configure --agent <name> updates the agent field in .partio/settings.json
• partio configure --strategy <name> updates the strategy field in .partio/settings.json
• Running partio configure without flags prints the current configuration
• The command returns a clear error if run outside a git repository or before partio enable
• Existing enable and disable commands are unchanged
• Table-driven tests cover flag parsing and settings file update

Program

See .minions/programs/add-partio-configure-command.md for implementation details.