Extract core simple contracts

[chubes4]

Summary

• Extract task input contracts and normalization into task-input.ts.
• Extract browser interaction script contracts and validation into browser-interaction.ts.
• Share the minimal object/list helpers through object-utils.ts while preserving public runtime-core barrel exports.

Verification

• npm run build
• npm run task-input-contract-smoke
• npm run browser-interaction-script-validation-smoke
• npm run command-registry-smoke
• npm run package-distribution-smoke

Tracking

• Closes Architecture cleanup: split runtime-core contracts, schemas, and runtime helpers #343
• Related: Architecture cleanup: deduplicate shared object, hash, and artifact manifest helpers #344
• Depends on merged Extract runtime command registry #348

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the focused runtime-core extraction, ran the requested verification commands, and prepared this PR for Chris to review.