XY-709: GitHub CLI authority P2: expose configured gh path and fallback diagnostics

[yvette-carlisle]

Summary

• add GitHub CLI command resolution tiers and project github.command_path authority
• surface non-secret GitHub CLI authority in status/diagnose evidence
• route PR inspection, review handoff validation, retained review readback, and GitHub operations through the same configured resolver
• document operator actions for missing or fallback-only gh authority

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test