docs: align documentation with current features

[anusbutt]

Summary

• align the benchmark spec with its recorded pre-fix and post-fix executions
• document all current CI platforms and add native PowerShell worked-example setup
• complete the README command inventory
• add a public reference for all 33 GraphKeeper diagnostics and six exit classes
• protect the diagnostic reference and README link with a documentation contract test

Verification

• focused contributor documentation tests: 11 passed
• complete npm test: 315 total, 312 passed, 3 expected platform-specific skips, 0 failed
• source diagnostic coverage: 33/33
• broken local Markdown links: 0
• git diff --check: passed