fix: skip token revocation when no token was acquired

[Dave-London]

Summary

• Adds steps.run.outputs.github_token != '' condition to the "Revoke app token" cleanup step
• Prevents confusing "Bad credentials" 401 error when the prepare phase fails before acquiring a token

Problem

The "Revoke app token" step runs with if: always(), meaning it executes even when the prepare phase fails (e.g., track_progress used with an unsupported event type). When no token was acquired, the curl sends an empty Authorization: Bearer header, producing a 401 "Bad credentials" error that confuses users debugging the actual failure.

Changes

• action.yml: Added steps.run.outputs.github_token != '' to the existing if condition on the "Revoke app token" step

Test plan

• TypeScript typecheck passes
• Prettier formatting passes
• When prepare fails: token output is empty -> step is skipped -> no 401 error
• When prepare succeeds: token output is populated -> step runs normally -> token revoked

Fixes #858