feat: support folders without git

[MaicolBen]

Summary

Closes #66.

• Adds a 'none' git isolation mode for projects that aren't git repositories
• Non-git folders can now be added as projects — the "Not a Git Repository" error dialog is removed
• isGitRepo is detected at project add/relink time and stored on the project
• When creating a task on a non-git project, git UI is automatically hidden (isolation selector, branch picker, branch prefix, merge/push buttons, changed files panel, diff viewer)
• "no git" badge shown in sidebar and title bar for these tasks
• Close dialog shows simple "stop agents" message without git warnings
• Fully backward compatible — existing projects default to isGitRepo: true

Test plan

• Add a non-git folder as a project (no error dialog)
• Create a task on a non-git project — git UI sections are hidden
• Agents spawn and run in the project folder
• Close task — simple close dialog, no git warnings
• Sidebar/title bar show "no git" badge
• Add a git folder — all git UI still works as before
• Persistence: close and reopen app, non-git project and tasks restore correctly
• Edit non-git project settings — git settings (branch prefix, delete branch, isolation, base branch) are hidden