test(e2e-command): cover fork commands in the platform harness

[vcauesantos]

Summary

• Port the fork design to the single-check Platform harness, which mirrors loft-enterprise: allow-forks, dispatch from the target branch for forks, the contract check on the workflow that will run, SHA-pinned checkout, and cache-mode: read with a cache probe.
• Add Platform scenarios to repro/fork-command/run.sh: two fork PRs with the same request both succeed, and a fork PR into a branch without the contract is refused.

This tests the loft-enterprise change before it is opened there.

Test plan

• actionlint clean, zizmor only flags the expected internal sticky-pr-comment/v1 tag
• shellcheck -x repro/fork-command/run.sh, parses under bash 3.2
• After merge: repro/fork-command/run.sh

Related to DEVOPS-1541