v4.2.6-beta

Added — compatibility mode graduation

• Compatibility mode graduation capability. Projects stuck in compatibility
  mode (stabilized-without-migration) can now graduate when they are already
  v4-compliant. Doctor detects graduation candidates at the maintenance route
  prompt and offers a one-click exit — a state-only write to sweetclaude.yaml
  with one-file blast radius, fully reversible via Doctor's restore flow.
• v4 compliance detection in characterize_project.py: new v4_compliance
  output block reports old-prefix count, v4-prefix count, required-field coverage,
  canonical-type compliance, duplicate-ID status, and standard structure.
• graduation-check and graduate CLI subcommands on recover_project.py
  for read-only validation and state-only graduation execution.
• graduation_candidate project shape and recover.graduate_from_compatibility
  capability in the manifest, with full safety contract (diagnose → validate →
  snapshot → approve → execute → verify → rollback).
• Doctor routes graduation-available before falling through to
  compatibility-mode, so compliant projects see the exit rather than the dead end.
• 17 new tests covering graduation-check happy path, blocker detection
  (old prefixes, duplicates, legacy types, parse errors), guard routing,
  doctor maintenance route, graduate execution, idempotency, and read-only
  guarantees.