Fixes rivet-dev#64 — exec() throws 'No shell available' when only NodeRuntime is
mounted (which registers 'node', not 'sh').
Before this change:
 createKernel() → mount(createNodeRuntime())
 kernel.exec('node -e "console.log(1)"') // throws: No shell available
After this change:
 - If 'sh' is registered: routes through shell (existing behavior)
 - If only 'node' is registered: parses command string, strips 'node'
 prefix, spawns node directly with remaining args
 - If neither: throws improved error with actionable guidance
Added:
- #parseCommandArgs(): shell-like tokenizer (handles '...', "..." escapes)
- #collectExecResult(): extracted common stdout/stderr collection logic
- 3 new test cases in kernel-integration.test.ts

...lyfills
Fixes rivet-dev#63 — adds ability to disable Node.js polyfill shims (fs, http,
process, Buffer, etc.) on globalThis.
When includeNodeShims: false is passed to createNodeRuntime():
- globalThis.fs, globalThis.http, globalThis.process, globalThis.Buffer
 are NOT injected into the isolate
- Useful for AI agents that need a clean globalThis scope
- fs/http are still accessible via require('fs') / await import('fs')
 when host filesystem is permitted via permissions
API:
 createNodeRuntime({ includeNodeShims: false })
Default: true (existing behavior unchanged).
Changes:
- packages/nodejs/src/kernel-runtime.ts: Added includeNodeShims to NodeRuntimeOptions
- packages/nodejs/src/driver.ts: Added includeNodeShims to NodeDriverOptions, pass to runtime config
- packages/nodejs/src/execution-driver.ts: buildFullBridgeCode() now keyed by includeNodeShims
 in a Map<boolean,string> cache; per-driver bridge code built in constructor
- packages/nodejs/test/kernel-runtime.test.ts: 3 new tests for includeNodeShims=false/true

Fixes rivet-dev#71
ssh2's isWritable() checks stream._readableState.ended === false.
Since the bridged net.Socket _readableState stub was missing the 'ended'
property, ssh2 treated all sockets as non-writable and never sent
handshake packets.