fix(sandbox): normalize node-pty path so daemon resolves on Windows

[Rooseveltfj]

What is this contribution about?

On Windows, running an agent fails immediately with
"could not derive node_modules path from node-pty resolution: C:...\node-pty\lib\index.js".

resolveNodePtyNodeModulesDir() resolves node-pty via Bun.resolveSync(), which
returns a backslash path on Windows, then searches for the /node_modules/
marker with lastIndexOf. The forward-slash marker never matches a backslash
path, so idx is -1 and it throws. The unix separator works only by accident.

Impact: the sandbox daemon never spawns, so agent execution is fully broken on
Windows — any prompt that triggers a tool fails.

Fix: normalize the resolved path to forward slashes before matching the marker.
The returned value feeds NODE_PATH, and forward-slash paths work on both Windows
and Unix. No-op on macOS/Linux (no backslashes to replace). The throw still shows
the original path for accurate diagnostics.

How to Test

1. On Windows, bun run dev, connect a model provider
2. Send a prompt that runs a tool (e.g. "write a song")
3. Expected: the agent executes and responds — no node-pty path error

Migration Notes

None.

Review Checklist

• PR title is clear and descriptive
• Changes are tested and working
• No breaking changes (no-op on macOS/Linux)

---

Summary by cubic

Normalize the resolved node-pty path to forward slashes so the sandbox daemon finds the node_modules directory on Windows. This fixes broken agent execution on Windows and is a no-op on macOS/Linux.

^{Written for commit 7a3f59b. Summary will update on new commits.}

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