Move SlashCommand::Plugins out of the 'unsupported resumed slash
command' catch-all and add a handler arm in run_resume_command that
calls handle_plugins_slash_command for list/help actions.
Mutation actions (install/uninstall/enable/disable) are rejected with
a clear error since there is no runtime to reload in resume mode.
Add /plugins coverage to resumed_inventory_commands test in
output_format_contract.rs: kind, action, reload_runtime, target.
Before: claw --resume session.jsonl /plugins --output-format json
-> {error: 'unsupported resumed slash command', type: 'error'}, exit 1
After: claw --resume session.jsonl /plugins --output-format json
-> {kind: 'plugin', action: 'list', ...}, exit 0
4fc63e338d