fix(start.mjs): remove chdir

[xav-ie]

What / Why / How

On startup the server changed its working directory into the plugin install
directory. That directory then leaks into any tool that reads a process's
working directory to find where the agent is working, for example a terminal
multiplexer opening a new split there instead of in the project. Nothing depends
on the change of directory, so remove it.

Affected platforms

Server startup path; client-agnostic. No behavior change to project resolution.

• All platforms

Test plan

A guard that the server no longer changes directory, and a check that project
resolution still returns the project when the process starts in the plugin
directory.

Checklist

• Tests added/updated
• npm test passes
• npm run typecheck passes
• Docs updated if needed (README, platform-support.md)
• No Windows path regressions (forward slashes only)
• Targets next branch (unless hotfix)