fix: stop modifying .gitignore after XDG state migration

[EugeneOsipenko]

Summary

• Remove ensureGitignoreEntries and sidecarGitignoreEntries from the gitstatus plugin — since state files moved to ~/.local/state/sidecar/, these entries (.sidecar/, .sidecar-agent, .sidecar-task, etc.) are no longer created in the project root
• Remove the startup hook in plugin.go:Start() that appended to .gitignore on every launch
• Remove the duplicate unused sidecarGitignoreEntries list from workspace/setup.go
• Remove now-obsolete gitignore tests

Test plan

• go build ./... passes
• go test ./internal/plugins/gitstatus/... passes
• Verify sidecar no longer modifies .gitignore when opening a project

🤖 Generated with Claude Code

[marcus]

Hey @EugeneOsipenko! Starling here (AI assistant on the project). 👋

This is a clean, well-scoped cleanup — removing the .gitignore-mutating code that became dead weight after the XDG state migration is exactly the right call. Sidecar has no business touching project root .gitignore files anymore.

Quick first read:

• Removing ensureGitignoreEntries and the startup hook in plugin.go:Start() is correct — those entries (.sidecar/, .sidecar-agent, .sidecar-task) don't land in the project root post-XDG, so appending them was silently polluting .gitignore on every launch
• Removing the duplicate sidecarGitignoreEntries list from workspace/setup.go is good housekeeping
• Deleting the now-obsolete gitignore tests is the right move (tests for removed behavior should go with the behavior)

Net result: -197 lines, zero new lines of feature code. The best kind of cleanup. 🧹

Flagging @marcus for a review pass — this one looks merge-ready to me, just want his sign-off on removing the test coverage. ✦