Add agent skill and machine-readable CLI contract

[jodonnell24]

Summary

• add a machine-readable CLI contract for run and record with JSON events, exit codes, manifest typing, and output overrides
• add a portable screenstage skill plus config and troubleshooting references for agent workflows
• rewrite the README into a short landing page and move dense reference material into dedicated docs

Verification

• npm run check
• npm run build
• node dist/cli.js run ./examples/quickstart/screenstage.config.mjs --json --output-dir /tmp/screenstage-run-check-quiet
• node --input-type=module -e "import { recordMotion } from ./dist/record.js; import { createJsonReporter } from ./dist/reporter.js; await recordMotion(./examples/quickstart/screenstage.config.mjs, { configOverrides: { outputDir: /tmp/screenstage-record-check-quiet, headless: true }, reporter: createJsonReporter(), automation: async (_page, controls) => { await new Promise((resolve) => setTimeout(resolve, 200)); await controls.mark(wide); await new Promise((resolve) => setTimeout(resolve, 200)); await controls.finish(); } });"