ci: add convergence claim-precheck workflow

[jeremylongshore]

Summary

• add the intent-eval-core caller for the canonical lab reusable gate
• pin the reusable workflow and canonical script to immutable commit SHAs
• run on main pushes and pull requests with read-only permissions

Verification

• YAML parse check
• actionlint
• pnpm run harness:verify
• pnpm run check reached the existing lint stage; it reports four pre-existing scripts/emit-evidence.ts unsafe-call errors unrelated to this workflow-only change
• git diff --check

Refs bead bd_000-projects-rqwk.9.2.

Closes #85.

[jeremylongshore]

Receipt: corrected the caller to the existing lab adapter commit 5a6946bfd59559db46afbdb37099fdd853d8561c after the initial nonexistent-ref dispatch was rejected before job creation. The canonical script remains pinned to 7b0bb069d7ce659bee760ac0a45ebdee42569841. The new gate / bd-claim-precheck check is green; native YAML/actionlint/harness checks pass.