Add deterministic CI checks classifier for maintenance automation

[Favour_emmanuel (Flave1)]

Description

Adds a deterministic CI checks classifier for maintenance automation.

• Normalizes states to: failed, pending, no_checks, policy_blocked
• Handles explicit gh pr checks output: no checks reported
• Adds script command: scripts/ci_checks_classifier.py
• Adds deterministic JSON schema output for machine consumption
• Adds unit + fixture tests for all required states
• Adds optional env-driven integration test for real-PR verification when available
• Adds usage docs in scripts/README.md
• Adds Makefile targets in root and libs/cli

Closes #6927.

Example output:

{
  "cancelled_count": 0,
  "checks_total": 0,
  "failed_count": 0,
  "merge_state_status": null,
  "no_checks_reported": true,
  "passing_count": 0,
  "pending_count": 0,
  "schema_version": 1,
  "skipped_count": 0,
  "state": "no_checks",
  "unknown_count": 0
}