feat(blackbox-acceptance-gate): add precondition check gate before document review

[forchain]

Summary

This PR updates the blackbox-acceptance-gate skill and repository guidelines:

1. Precondition Check Gate (1.5 Precheck Dependencies):

   • Added a mandatory precheck phase to blackbox-acceptance-gate before document review (2. Lock Approval).
   • Requires non-mutating, read-only diagnostic probes for external resources (databases, log files, third-party APIs / exchange endpoints, credentials, CLI binaries).
   • Generates precondition-report.md as evidence.
   • Enforces a hard block: document review cannot be initiated if any required precondition is failed or unclear.

2. Repository Agent Guidelines (AGENTS.md):

   • Recorded the iron rule requiring agents to read remote.origin.url to resolve the project's bound GitHub account (forchain) before Git operations.