fix: exit with error when --continue finds no sessions

[Kushalk0677]

When starting opencode on a project with no sessions, using --continue/-c errors with an invalid dummy sessionID and leaves the TUI in a broken state.

Issue for this PR

Closes #25989

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When --continue is used, the session list is now checked before the TUI starts. If no root sessions exist, it exits with a clear error message instead of entering the TUI with an invalid dummy sessionID that fails validation.

How did you verify your code works?

• Confirmed validate-session.ts checks for root sessions when --continue is passed
• Verified the error message prints and exits before the TUI renders

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found one potential duplicate:

Related PR:

• fix(opencode): scope continue sessions to root worktree #26011 - fix(opencode): scope continue sessions to root worktree - fix(opencode): scope continue sessions to root worktree #26011

This PR appears related because it also deals with the --continue flag and session handling. Both PRs are addressing issues with the --continue functionality, though #26011 focuses on scoping sessions to root worktree while #26014 (the current PR) specifically fixes the error exit behavior when no sessions are found.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Kushalk0677]

Hi @rekram1-node, this fixes the --continue issue where the TUI crashes with an invalid sessionID when there are no prior sessions. It validates before the TUI starts and exits cleanly.

[rekram1-node]

Appreciate the PR but opencode already behaves as expected no?

[rekram1-node]