chore: add format scripts and remove unused eslint-plugin-unused-imports

[pyramation]

Summary

This PR builds on the ESLint config simplification by adding the missing Prettier workflow scripts and removing a dead dependency.

Changes:

• Added format script (prettier . --write) for formatting all files
• Added format:check script (prettier . --check) for CI validation
• Added fix script that runs ESLint fixes across all packages, then formats with Prettier
• Removed eslint-plugin-unused-imports from devDependencies (was removed from ESLint config in parent PR but dependency remained)

Review & Testing Checklist for Human

• Run pnpm fix to verify the combined lint+format workflow works correctly
• Run pnpm format:check to verify it correctly detects unformatted files
• Confirm Prettier respects .gitignore and doesn't format node_modules/, dist/, etc. (no .prettierignore exists, so it relies on gitignore)

Test plan: Run pnpm format:check - it should report formatting issues. Then run pnpm fix - it should lint and format. Run pnpm format:check again - it should pass.

Notes

This PR targets the feat/eslint-config-12-25 branch (parent PR #479), not main.

Link to Devin run: https://app.devin.ai/sessions/827798cfe6ca476795794f7539cb234c
Requested by: Dan Lynch (pyramation@gmail.com) / @pyramation

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