feat(dsl): add upload action for file input via CDP setFileInputFiles

Summary

• Adds StepUpload type to the DSL for file upload actions via CDP DOM.setFileInputFiles
• Adds upload to VALID_ACTIONS, KNOWN_STEP_KEYS, and validateStep in config.ts
• Implements case "upload": in runner.ts using existsSync validation + CDP file input
• Adds formatStep support for human-readable upload step display
• Includes unit tests for config validation and runner execution

Use Case

Enables WebReel tutorial scripts to automate file uploads (e.g., uploading audio/document files) as part of recorded browser demos.

Test plan

• Config validation tests pass for upload action
• Runner test validates CDP setFileInputFiles call
• bun test passes

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