feat(github): add review thread replies

[thomaseude]

Issue for this PR

Closes #26691

Type of change

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

What does this PR do?

Adds an opt-in reply_to_review_thread input for the GitHub Action.

When enabled and opencode is triggered from a pull_request_review_comment event, the action creates its working/final response as a reply in the triggering review comment thread instead of posting a top-level PR comment. Existing issue and PR comment behavior remains unchanged by default.

How did you verify your code works?

• Ran git diff --check
• Reviewed the GitHub REST calls used for creating and updating pull request review comment replies

I could not run the full Bun typecheck locally because Bun is not installed in this environment.

Screenshots / recordings

Not applicable. This is a GitHub Action behavior change, not a UI change.

Checklist

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

[github-actions]

Hey! Your PR title Add review thread replies for GitHub action doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[thomaseude]

Related context I found while checking for overlap:

• [FEATURE]: Show GitHub PR review comments inline in the Desktop review panel #18964 is open and covers surfacing GitHub PR review comments inline in the Desktop review panel. This PR is adjacent but focused on the GitHub Action response location, not the Desktop UI.
• [FEATURE]: Support inline PR review comments in GitHub Action #13918 is closed and requested native inline PR review comments from the GitHub Action. This PR is narrower: it does not generate a full inline review, it only lets opencode reply in the triggering review comment thread when invoked from a pull_request_review_comment event.
• OpenCode github action: Add support for pull_request_review and pull_request_review_comment as triggering events #2152 is closed and covered support for pull_request_review_comment triggering. This PR builds on that event support by improving where the action posts its response.

[github-actions]

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