fix: print empty session json

[he-yufeng]

Issue for this PR

Fixes #30100

Type of change

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

What does this PR do?

session list --format json printed empty stdout when no sessions existed, which makes the command invalid for scripts expecting JSON. This keeps the output machine-readable by returning [] for the empty list case and adds a CLI smoke regression.

How did you verify your code works?

• bun --cwd packages\opencode test test\cli\smokes\read-only.test.ts -t "session list json"
• bun --cwd packages\opencode typecheck
• bunx prettier --check packages\opencode\src\cli\cmd\session.ts packages\opencode\test\cli\smokes\read-only.test.ts
• bunx oxlint packages\opencode\src\cli\cmd\session.ts packages\opencode\test\cli\smokes\read-only.test.ts (existing warning only, no errors)
• git diff --check

Screenshots / recordings

N/A; this is CLI JSON output behavior.

Checklist

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

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.