[FEATURE]: Add --restore-last-session CLI flag (and/or config option) to resume the last session on startup #40661

Problem

When reopening opencode in a project directory, there is no way to automatically resume the last session. You have to manually navigate to the session list and pick it each time.

Proposed Solution

Add a --restore-last-session CLI flag that resumes the most recently active session for the current working directory on startup:

opencode --restore-last-session

Optionally, expose the same behaviour as a config option so it can be set permanently:

{
  "$schema": "https://opencode.ai/config.json",
  "restore_last_session": true
}

When enabled, opencode would look up the most recent session associated with the current directory and open it directly, skipping the new-session default.

Alternatives Considered

• Manually selecting a session via the session list on each launch — works but requires extra steps every time.
• Wrapping the CLI in a shell alias that resolves the last session ID and passes it via a hypothetical --session <id> flag — fragile and depends on internal storage details.

Use Case

Useful for anyone who works on a single project at a time and wants to pick up exactly where they left off without extra navigation.