[FEATURE]: Factory reset, session management, and cache cleanup tools #25816

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

A set of session and data management tools to give users more control over their local data.

1. Factory Reset / Reset to Initial State
Add an option to reset OpenCode to its initial installation state, clearing all configuration, cached data, sessions, and user data. Useful for troubleshooting, preparing for reinstall, or ensuring a complete cleanup before handing off a machine.

2. Bulk Session Management

• Delete all sessions in one action (currently users must delete sessions one by one)
• Delete individual sessions (no clear way to remove unwanted/expired sessions, related to [FEATURE]: Session Lifecycle Management — unified storage reclamation, auto-archival, and child session cleanup #16101)
• Session list gets cluttered over time with no cleanup mechanism

3. Local Cache Cleanup
A command or option to clear locally cached data (model lists, provider responses, etc.) without affecting session history or configuration. Useful when cache becomes stale or takes up too much space.

About the duplicate flag: This request covers THREE features (factory reset, bulk session management, cache cleanup). #21619 covers only individual/conversation deletion. #8677 covers only a /delete command for the current session. Neither covers factory reset or cache cleanup, which are the primary requests here.