github: only build on PR approval, skip on raw push/open.

[Clement-Wang26]

Event	Conflict check	Sensitive file check	Format check	Build
workflow_dispatch	—	—	—	✅ Always
push to main	—	✅ Fail if hit	—	❌ Never
pull_request	✅ Silent skip	✅ Fail if hit	✅ Always	❌ Never
pull_request_review	✅ Silent skip	✅ Silent skip	—	✅ First approval
issue_comment /build	✅ Silent skip	✅ Silent skip	✅ Yes	✅ Yes

Legend:

• Silent skip = exits with ✅, no build
• Fail = exits with ❌, blocks merge"