fix: validate release app credentials early

[vitormattos]

Fixes the failure mode observed in LibreSign Prepare release run 35667899070.

• validates GitHub App id/private key before expensive release planning or post-merge restoration;
• reports actionable consumer configuration errors instead of the indirect Octokit appId option is required failure;
• fixes Markdown code formatting in the authorization job summary so Bash does not execute values inside backticks;
• documents that write-capable consumer workflows need the App variable/secret in their own Actions context and that the App installation must include the consumer repository;
• bumps the workflow platform patch version to 0.6.2.

No credential values are logged.