fix(ci): surface missing baseline in PR comment

[veksen]

Summary

• When no baseline exists on the comparison branch, the PR comment now shows a callout explaining how to establish one by adding a push trigger
• Adds an informational log message in CI output when no baseline is found
• Updates README setup steps to include workflow trigger configuration with both pull_request and push to main

Test plan

• Reporter unit tests pass (17/17)
• Verify PR comment renders the baseline callout when no previous run exists
• Verify the log message appears in CI output when comparison branch has no runs

Note: the site docs (apps/docs/src/content/docs/guides/ci-integration.md) need a matching update — see .context/handoff-docs-baseline.md for details.

🤖 Generated with Claude Code

[github-actions]

Query Doctor Analysis

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28 queries analyzed

2 pre-existing issues

• SELECT "guests"."id", "guests"."session_id", "guests"."username", "guests"."avatar_path", "guests"."color", "guests"."side", "guests"."audio_recording_path", "guests"."audio_recording_public", "gue...
  index assets(event_id, uploader_id, inserted_at desc)
  cost 15,922 → 1,639 (90% reduction)
• SELECT * FROM guest_ip_addresses WHERE ip_address = '127.0.0.1';
  index guest_ip_addresses(ip_address)
  cost 126 → 9 (93% reduction)

_{Using assumed statistics (10000 rows/table). For better results, sync production stats.}