feat: support non-git projects

[klioen]

Summary

• allow opening existing non-Git directories as first-class projects without initializing Git
• preserve Files, editing, Terminal, Chat, settings, durable restore, and filesystem watching
• hide Changes, diff, branch, and managed Worktree UI for directory projects
• enforce Git and managed Worktree capability checks in Electron Main
• migrate project state from v2 to v3 with existing projects deterministically marked as Git
• keep file lists visible while high-frequency filesystem events refresh in the background

Security and compatibility

• Renderer continues to use opaque workspace IDs and relative paths; no arbitrary-path IPC was added
• Git/Worktree capability is derived from the durable Main catalog rather than Renderer input
• directory watchers emit filesystem events only and never Git invalidations
• existing Git project IDs and behavior are preserved during migration
• SpireCode only opens existing directories; no New Project/directory-creation flow is added

Verification

• pnpm check
• 63 Vitest files, 409 tests passed
• release tests passed
• macOS app and DMG smoke tests passed
• installed app signature and app.asar hash verified locally