ci: split pre/post-merge jobs in test.yml

[fullstackjam]

Summary

• Move contract schema (L2), curl|bash smoke, and old-cli compat to post-merge only (push / dispatch events) — they're too slow or network-dependent to block every PR
• PRs now only require lint and unit (L1), which are the fast checks
• Update required-checks.txt to match (two entries instead of five)
• Update MERGE_POLICY.md with a pre-merge / post-merge split table

The three heavy jobs still run on every merge to main and on workflow_dispatch / repository_dispatch — they just don't block PR feedback.

Test plan

• Verify pre-merge CI on this PR only runs lint and unit (L1)
• After merge, verify contract schema (L2), curl|bash smoke, old-cli compat all run on the push-to-main event
• Update live branch protection to match required-checks.txt (remove the three heavy checks from required status checks):

  gh api repos/openbootdotdev/openboot/branches/main/protection \
    --jq '.required_status_checks.contexts'
  # then update via gh api -X PUT per docs/MERGE_POLICY.md