add cmd option for prise.spawn

[jossephus]

add cmd option for prise.spawn enabling execution of a command from prise.spawn.

  prise.spawn({ cmd = {  "npm run dev" } })

Closes #65 with a slight difference when specifying the command.

[github-actions]

Review Summary

The implementation adds a cmd parameter to spawn PTY requests, allowing an initial command to be executed. The overall structure is clean and follows the existing patterns well.

Issues found:

1. Partial write risk (server.zig:2273): The return value of posix.write is discarded, which could result in truncated commands for longer inputs.
2. Potential race condition (server.zig:2269): The command is written immediately after PTY creation, but the shell may not be fully initialized yet.

The client-side changes and UI integration look correct.

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Review context: https://ampcode.com/threads/T-019bafe3-3f81-756d-b252-899591d296aa