chore: prepare release v0.4.0

[sjnims]

Release v0.4.0

Highlights

• Query Termination: Query.close() for forceful query termination
• Comprehensive Cost Tracking: Stage-level cost breakdown across all pipeline stages
• Bug Fixes: YAML null handling, cost estimation formula, plugin load costs

Changes

See CHANGELOG.md for full details.

Checklist

• Version updated in package.json
• CHANGELOG.md updated with release notes
• All CI checks pass locally
• Tested locally

[claude]

Claude finished @sjnims's task in 52s —— View job

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PR Review: Release v0.4.0

• Run quality checks (typecheck, lint, format, knip)
• Review the diff for all changed files
• Provide feedback summary

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✅ Looks good

This is a clean release preparation PR.

Quality Checks

Check	Status
npm run typecheck	✅ Pass
npm run lint	✅ Pass
npm run format:check	✅ Pass
npm run knip	✅ Pass

What's Good

• Version bump is consistent across package.json and package-lock.json (0.3.0 → 0.4.0)
• CHANGELOG follows Keep a Changelog format with proper sections (Added, Changed, Fixed)
• PR references included in changelog entries for traceability (feat: add Query.close() for forceful query termination #340, fix: normalize YAML null values to undefined before config validation #339, fix: correct execution cost estimation formula #332, etc.)
• Comparison links updated correctly at the bottom of CHANGELOG.md
• Date format is correct (2026-01-22)

Changes Summary

The PR updates:

1. Version to 0.4.0 in both package files
2. CHANGELOG.md with new release notes documenting:
   • Query termination feature (Query.close())
   • Comprehensive cost tracking across all stages
   • Bug fixes for YAML null handling and cost estimation

No code changes, tests, or type definitions are modified - this is purely a release preparation commit.

Ready to merge. 🚀