feat(cli): add workflow validate command for CI checks

[EisonMe]

Summary

Add a new CLI command keep workflow validate to validate Keep workflow YAML files locally and in CI.

What's included

• New command: keep workflow validate
• Supports --file/-f for single workflow file validation
• Supports --dir/-d for directory batch validation
• Prints validation summary with passed/failed counts
• Exits with code 1 when any file fails (CI-friendly)
• Works without API key

Tests

• Added unit tests for:
  • single-file success
  • single-file failure
  • directory mixed results

Docs

• Added docs page for workflow validate
• Updated workflow CLI docs index

Fixes #3376
Related issue: #3376

[vercel]

@EisonMe is attempting to deploy a commit to the KeepHQ Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

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[greptile-apps]

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