Summary

• Promote copyIgnoredOnWorktreeCreate, copyUntrackedOnWorktreeCreate, and pullRequestMergeStrategy from repo-only settings to global defaults with per-repo optional overrides (nil = use global).
• Add new "Copy ignored/untracked files" toggles in Settings > Worktrees and a "Pull Requests" section with merge strategy picker in Settings > GitHub.
• Convert per-repo toggles to three-way pickers showing "Global (value)" when defaulting.
• Fix repo default directory prompt to show the resolved path (including repo name) instead of the generic repos directory.

Test plan

• Open Settings > Worktrees — verify new copy-ignored and copy-untracked toggles appear.
• Open Settings > GitHub — verify "Pull Requests" section with merge strategy picker appears after "GitHub CLI".
• Open Settings > [repo] — verify copy toggles and merge strategy show as pickers with "Global (value)" option.
• Change global copy-ignored to true, verify repo picker updates to show "Global Yes".
• Set a repo override to "No", create a worktree, verify the override is respected.
• Set merge strategy globally to "Squash", verify a repo with "Global" uses squash when merging.
• Verify existing repos with previously saved settings retain their explicit values.
• Verify the repo default directory prompt shows the resolved path (e.g. ~/.supacode/repos/my-repo/).

Closes #144