feat(platforms/browser): just-bash integration — full Workspace parity in browser

[arjunkmrm]

Summary

Wires `just-bash` (Vercel Labs' in-browser bash interpreter + VFS) into the browser platform. `` now works in browser context with all five tools — same parity as Node and Bun, except just-bash is a "broad subset" of POSIX bash rather than a full system shell (no system binaries, no real subprocesses).

What ships

• `src/platforms/browser/just-bash.ts` — wraps just-bash's `FileSystem` as Effect's `FileSystem.FileSystem` and just-bash's `Bash` as Effect's `CommandExecutor.CommandExecutor`. Both wrappers share one `Bash` instance per layer, so the VFS stays consistent across fs-tool calls and bash-tool calls.
• `justBashPlatform(opts?)` — composed layer (FileSystem + Path + CommandExecutor) ready to pass as `createAgentRuntime({ platform })`. Accepts `files` / `env` / `cwd` options forwarded to the just-bash `Bash` constructor.
• `src/platforms/browser/index.ts` — exports `justBashPlatform` alongside the existing `partialPlatform` (HttpClient only) for users who don't want shell.
• `just-bash` added as peer-optional + devDep.

How non-`bash` commands work

Commands like `Command.make("grep", "-rIn", ...)` are reassembled via single-quote shell-escaping and run as a single script through `bash.exec()`. just-bash bundles a `grep` built-in so Workspace's grep works against the VFS. `PipedCommand` (`a | b`) glues with `|` and dispatches the same way.

Usage

```tsx
import { createAgentRuntime } from "@flamecast/agentjsx"
import { justBashPlatform } from "@flamecast/agentjsx/platforms/browser"
import { Agent, Block, Workspace, Messages } from "@flamecast/agentjsx/components"

const agent = createAgentRuntime({
platform: justBashPlatform({ files: { "/README.md": "# In-browser workspace" } }),
infer: fetchBackedInferFn,
context: () => render(

…



),
})
```

Test plan

• `pnpm run check-types` clean
• `pnpm run test` — 160/160 passing (159 + 1 new integration test that exercises write_file → read_file → bash → list_dir, proving the VFS is shared across fs and bash tools)
• Manual browser test — would need a real bundler + browser to verify the integration genuinely runs in browser context. Test runs in Node since just-bash works there natively; the browser-specific concern is bundler compat (no Node-only fs/child_process imports leak in), which we'd verify with a tree-shake test in a follow-up.