feat(js): add customBuiltins to Bash and BashTool

[mayneyao]

Python has Bash(custom_builtins={...}) from #1315 but the JS bindings
(@everruns/bashkit) were missing this feature.

What was missing

• Bash has persistent VFS across execute() calls but no way to
  register JS callbacks as persistent builtins
• ScriptedTool.addTool() has callbacks but VFS is ephemeral per call
• The issue described JS bindings already have this via Bash.defineCommand()
  but in reality defineCommand never existed

What was implemented

• BashOptions.customBuiltins: Record<string, BuiltinCallback> — constructor-time
• Bash.addBuiltin(name, callback) — post-construction
• BashTool equivalent support
• BuiltinContext with name, argv, stdin, env, cwd
• 10 tests covering pipes, VFS persistence, reset survival, pipelines

import { Bash } from '@everruns/bashkit';

// Needed API that was missing
const bash = new Bash({
  customBuiltins: {
    'get-order': (ctx) =>
      JSON.stringify({ id: ctx.argv[0], status: 'shipped' }) + '\n',
  },
});

await bash.execute('mkdir -p /scratch');
await bash.execute('get-order 42 > /scratch/order.json');     // → VFS
const result = await bash.execute('cat /scratch/order.json');  // persists across calls

Notes

• executeSync() deadlocks with custom builtins (same limitation as
  ScriptedTool). Use execute() (async).
• Custom builtins survive reset() but are host-side config — pass
  customBuiltins again on snapshot restore.

[mayneyao]

I am migrating from just-bash to bashkit. Need persistent VFS + custom builtins together — but Bash/BashTool JS bindings were missing this. Python got it in #1315 (constructor-time custom_builtins dict). JS only had ScriptedTool.addTool() which creates a fresh interpreter per call (no VFS persistence across execute()). So I opened a PR to add customBuiltins + addBuiltin() to the JS bindings.

@chaliy