fix: reply-to-mention doesn't work for review comments on forks

[derekmisler]

Summary

Fixes @docker-agent mentions not working for review comments on fork PRs. The issue was that the mention-reply handler couldn't access the original comment context when triggered from a fork, since fork workflows have limited permissions. This PR adds a cross-run artifact mechanism to pass comment context from the trigger workflow to the reply handler.

Changes

• Extract and expose comment metadata in resolve-context job: in_reply_to_id, has_mention, is_review_cmd, and author_type are now available as workflow outputs for downstream jobs to make routing decisions.

• Add cross-run artifact support in reply-to-mention job: Downloads the PR review context artifact (saved by the trigger workflow) when invoked from a fork, allowing access to the original comment even with limited permissions.

• Synthesize mention-reply event context: Reconstructs a synthetic pull_request_review_comment event from the artifact, enabling the mention-reply handler to process fork comments as if they were native events.

• Improve routing logic: Updated conditions to distinguish between replies to agent comments (handled by reply-to-feedback) and top-level mentions (handled by reply-to-mention), with proper support for both direct events and cross-run artifact scenarios.

• Update documentation: Clarified that @docker-agent mentions now work in both PR-level comments and inline review comments, including on fork PRs.

Closes: https://github.com/docker/gordon/issues/559

[derekmisler]

/review