fix: stop browse editing tracked .gitignore

[fedster99]

Summary

• stop browse from mutating the tracked project .gitignore as part of startup
• ignore .gstack/ through repo-local Git exclude metadata instead, including worktree repos
• move the ignore write into the locked CLI startup path and harden the config tests

Problem

browse was appending .gstack/ to the tracked .gitignore file during startup. That left surprise worktree diffs in user repos, and because the write happened outside the startup lock it could race and append duplicate .gstack/ lines.

Fix

• split state-dir creation from Git ignore mutation
• resolve the repo-local exclude path for both normal repos and worktrees
• write .gstack/ to Git exclude metadata from the locked CLI path only
• make the config tests self-contained with temp git repos so they pass outside a git checkout too

Verification

• bun test browse/test/config.test.ts
• bun run build