feat(validate): add --json output for machine-readable validation

[sonwr]

Summary

• add --json support to heymark validate for machine-readable output
• include structured fields: valid, skillCount, and errors[] entries with tool, path, error
• keep existing human-readable validate output unchanged when --json is not passed
• update help text and README with validate --json usage and example
• add integration tests for valid/invalid JSON validation flows

Testing

• npm test
  • pass: validate --json returns structured errors and exit code 1 when invalid
  • pass: validate --json returns valid=true and exit code 0 when valid

Notes

• this PR is stacked on top of feat/validate-and-dry-run (feat: add validate command and dry-run mode for sync/clean #8) to keep the diff focused on JSON output only.