[FEATURE]: Add TUI ability to inspect session system messages #24990

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

Currently, there is no way to directly inspect or view system messages (such as system prompts, environment info, or initial context details) from the OpenCode TUI during a session. This feature request proposes adding a TUI feature to view the current or previous system message(s) associated with the active session or a selected session.

Benefits:

• Allows users to verify, debug, and audit what system prompt/context is being provided to the model.
• Helps troubleshoot session behavior by letting users see key context details (like working directory, platform, custom environment settings, etc.) used in system messages.
• Improves transparency for users who suspect system context is affecting completions or need to confirm OPENCODE's understanding of environment state.

Suggested implementation:

• Add a TUI shortcut or command (e.g., /system or a context menu option in the session list) that opens the latest system message for the current session.
• Optionally support viewing system messages for previous sessions or the full session timeline.
• Display the exact content of the message (with redaction as needed for sensitive values), formatted for readability.

This would give TUI users parity with API/web users who can already view or export system messages, and is especially helpful for CLI-only workflows.

Extra points for being able to edit them in EDITOR for while that part stays alive, but that could conflict with how it is implemented so probably not super important. But it would be really nice for testing small system message changes.