Add lint command

[codeaholicguy]

Summary

Adds a new ai-devkit lint command to validate AI DevKit workflow readiness, with feature-aware checks and CI-friendly output.

Scope

• Introduces ai-devkit lint for base workspace validation (docs/ai/*/README.md)
• Adds --feature mode to validate:
  • feature docs across lifecycle phases
  • feature branch/worktree conventions
• Supports feature normalization (foo and feature-foo)
• Adds --json output for machine-readable CI integration
• Enforces exit codes:
  • 0 when required checks pass
  • 1 when required checks fail
  • missing dedicated worktree is warning-only

Implementation

• Command entrypoint in commands/lint.ts
• Business logic in services/lint/lint.service.ts
• Rule-based lint architecture in services/lint/rules/* for scalability
• Shared git helpers in util/git.ts (safe, argument-based execution)
• Terminal output uses existing terminal-ui patterns

Testing & docs

• Added command-level and service-level tests
• Added rule-level tests for base docs, feature name validation, and git/worktree behavior
• Updated CLI README with lint usage examples

Validation

• nx run cli:test -- --runInBand lint.test.ts passed
• npm run lint passed with existing unrelated warnings only