feat(cdk): add 4 additional github:* resource tags

[mayakost]

Summary

• Adds 4 new github:* tags to complement the 9 tags from feat(cdk): add github:* resource tagging strategy #85, bringing the total to 13
• Same pattern: injected via CDK context (-c github:key=value), defaulting to none
• Tests updated to cover all 13 tags

New Tags

Tag key	CI variable	Rationale
github:base-ref	${{ github.base_ref }} / event.merge_group.base_ref	Distinguishes deployment target branch from source. Critical for merge_group and pull_request where github:ref is the source, not the target.
github:pr-number	${{ github.event.pull_request.number }} / extracted from merge group ref	Direct trace-back from deployed resource → PR. Simplifies cleanup queries (aws resourcegroupstaggingapi get-resources --tag-filters Key=github:pr-number).
github:run-attempt	${{ github.run_attempt }}	Distinguishes retries (same run-id, different attempt). Without this, redeployed resources from a retry look identical to the first attempt.
github:workflow	${{ github.workflow }}	If multiple workflows can deploy (e.g., deploy.yml vs hotfix.yml), identifies which produced this state.

Test plan

• 3 existing tests in cdk/test/stacks/github-tags.test.ts updated for all 13 tags
  • All 13 tags default to none when no context provided
  • Tags reflect context values when provided (including new tags)
  • Empty string values resolve to none
• TypeScript compiles cleanly

[scottschreckengaust]

LGTM