Replace recorded agent replay with a standalone JavaScript script runtime.
Install synchronous agent primitives in an isolated V8 context, add console
output, return structured extract values as JS objects/arrays, and route script
execution through the new runtime.
Update recording to emit .js function calls, default /save filenames to .js,
drop script-level extract save support, and refresh agent docs/tutorials for
the new format.
Self-healing is disabled for now.
