feat: add metadata JSON readiness output

[panva]

This adds structured readiness output to git node metadata --json so automation can consume PR landability checks without matching human-readable CLI output.

The JSON output includes:

• ready
• readiness
• exitCode
• metadata
• reasonCodes
• reasons

It also adds stable reason codes from PRChecker for the existing readiness checks.

Exit Codes

git node metadata --json uses category-level exit codes:

• 0: PR is ready
• 20: PR is not ready for deferrable metadata reasons
• 40: PR is not ready for a hard or mixed reason
• 1: unexpected tool/runtime failure

20-29 are reserved for future deferrable metadata states, and 40-49 are reserved for future hard metadata failure states.

The current deferrable set is scoped to the lightweight commit-queue selector use case from nodejs/node#64343:

• missing-approval
• missing-tsc-approval
• wait-time

[codecov]

Codecov Report

❌ Patch coverage is 87.54579% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.44%. Comparing base (9250fba) to head (d416c6a).

Files with missing lines	Patch %	Lines
lib/pr_checker.js	92.63%	12 Missing ⚠️
components/git/metadata.js	67.64%	11 Missing ⚠️
components/metadata.js	85.52%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1113      +/-   ##
==========================================
+ Coverage   62.01%   62.44%   +0.43%     
==========================================
  Files          46       48       +2     
  Lines        7505     7884     +379     
==========================================
+ Hits         4654     4923     +269     
- Misses       2851     2961     +110     

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