Pin proof-pr v0.2.6

[saagpatel]

Summary

• pin the proof-pr reusable workflow and CLI install ref to v0.2.6
• picks up advisory receipt-hygiene job-summary output from the reusable workflow

Proof

• proof-pr v0.2.6 release: https://github.com/saagpatel/proof-pr/releases/tag/v0.2.6
• bridge-db dogfood run passed with the new Receipt hygiene step: https://github.com/saagpatel/bridge-db/actions/runs/27870503391

Risk

• Low: workflow pin-only change. The new hygiene step is advisory by default and does not block merges unless strict mode is enabled later.

Rollback

• Revert this workflow pin to v0.2.5 if the reusable workflow regresses in this consumer.