feat(worktree): repo-local random worktrees and default repo session worktrees

[galdawave]

Summary

• Store newly created worktrees under each repo at <repo>/.idx0/worktrees/ instead of app support.
• Use random worktree directory names (wt-<token>) with collision-safe suffixing.
• Default repo-backed session creation to always create a worktree, with an opt-out settings toggle.
• Keep repo status clean by idempotently adding .idx0/ to local git exclude.

Details

• WorktreeService now creates worktree paths inside repo-local .idx0/worktrees and resolves .git directory or .git file (gitdir:) when updating local exclude.
• SessionService.createSession now auto-creates worktrees for repo-backed sessions when defaultCreateWorktreeForRepoSessions is enabled (default true), while still allowing explicit attach/create flows.
• New Session UI reflects enforced behavior by disabling the worktree toggle when the setting is enabled for a git repo and shows guidance to disable in Settings.
• Session settings copy was updated to match the new behavior.
• Integration and unit tests were expanded for random path generation, collision handling, .idx0/ exclude idempotence, and default session behavior.

Related Issues

• Related to workspace-local random worktree rollout.

How to Validate

1. Run focused tests:
   • xcodebuild -project idx0.xcodeproj -scheme idx0 -destination 'platform=macOS' test -only-testing:idx0Tests/WorktreePathGenerationTests -only-testing:idx0Tests/SessionServiceIntegrationTests
2. In app Settings > Sessions, keep "Create Worktree By Default" enabled.
3. Create a session with a git repo folder and verify session is worktree-backed under <repo>/.idx0/worktrees/wt-*.
4. In that repo, run git status --short and verify .idx0/ is not shown as untracked.
5. Disable "Create Worktree By Default" and create another repo session; verify non-worktree branch mode is available and session can be created without a worktree.

Pre-Merge Checklist

• lint-docs check is green
• tests check is green
• maintainability result reviewed (phase 1 report-only)
• Added/updated tests (if needed)
• Updated relevant documentation and README (if needed)
• Noted breaking changes (if any)
• Tested on macOS