What

Adds CreateVmConfig.jsRuntime so callers can choose which JavaScript host environment guest code sees — modeled on esbuild's platform. Omitting it is byte-for-byte today's behavior (full Node.js emulation).

config: { jsRuntime: { platform: "browser", moduleResolution: "node" } }

• platform: node | browser | neutral | bare (esbuild's three + a sanctioned language-only tier)
• moduleResolution: node | relative | none (independent axis)
• allowedBuiltins: tri-state Option — omitted = engine default, [] = deny all, [..] = exactly those (node platform only)

How

• The guest JS surface is baked into a shared V8 snapshot, so non-node platforms are produced by subtractively scrubbing baked globals in the per-execution runtime shim (prepend_v8_runtime_shim) — not by gating the config-blind setupGlobals().
• Builtin denial is enforced at the bridge's rejectRestrictedBuiltinRequest sink (the chokepoint for import/import()/require), via a one-shot __agentOsInitJsRuntime init fn that stores the allow-list in a bridge closure and is deleted by the shim — no reachable __agentOs* global remains.
• moduleResolution is gated at the Rust resolver (MODULE_NOT_FOUND for denied bare/relative); builtin denial surfaces uniformly as ERR_ACCESS_DENIED.
• browser narrows crypto to WebCrypto (no randomBytes) and keeps npm/ESM→CJS interop working (module machinery _requireFrom/_moduleModule is governed by moduleResolution, not stripped by platform).
• Identity leak surfaces (_processConfig, __agentOsProcessConfigEnv) are scrubbed for non-node platforms.
• Removes the legacy configure_vm.allowed_node_builtins field end-to-end (BARE schema → generated wire types → ~40 call sites).

Docs & spec

• New docs page Features → Runtime & Platform (website/src/content/docs/docs/features/runtime-platform.mdx) with the full matrix + examples.
• Design spec in specs/js-runtime-platform.md (peer-reviewed; the wiring reflects the corrected mechanism).
• CLAUDE.md: esbuild-vocabulary rule, with bare documented as the one sanctioned extension.

Testing

• vm-config: 41 unit tests (validation, tri-state, every platform × moduleResolution round-trip).
• Pentest suite (crates/execution/tests/javascript_v8.rs, in javascript_v8_suite) proving the surface is actually disabled: under bare, process/Buffer/require/node:*/timers/console/identity channels are unreachable (incl. via the Function constructor) while language + WebAssembly remain; browser exposes web globals, denies node:fs with ERR_ACCESS_DENIED, loads a CJS npm package, and keeps fetch callable; moduleResolution: none/relative deny imports as specified; node allowedBuiltins:["path"] permits path, denies fs.
• TS check-types, full sidecar lib+tests build, and pnpm --dir website build all green.