feat: support existing branches when adding worktrees

[nelson870708]

Pull Request

1. Summary

Support creating worktrees from existing branches in addition to creating a new branch. The add-worktree modal now separates existing-branch and new-branch flows, filters out branches already checked out in worktrees, and uses cleaner default worktree paths based on the main worktree.

2. Related Issue

• NA

3. Changes

Extension (src/)

• src/extension.ts
  • Resolve the main worktree path before building the default add-worktree folder.
  • Default new worktrees under <main-repo>.worktrees.
• src/__tests__/extension.test.ts
  • Add coverage for add-worktree default path resolution when the active repo is a linked worktree.

Webview (webview-ui/)

• webview-ui/src/components/modals/AddWorktreeModal.svelte
  • Add separate modes for using an existing branch and creating a new branch.
  • Show only the existing branch selector in existing-branch mode.
  • Show Start at and branch name fields only in new-branch mode.
  • Filter out branches already checked out in worktrees.
  • Generate default worktree folders as child paths under the default base folder.
• webview-ui/src/components/modals/__tests__/AddWorktreeModal.test.ts
  • Update modal tests for the new mode selection behavior.
  • Add coverage for branch filtering and default folder path generation.
• webview-ui/src/lib/i18n/en.ts
• webview-ui/src/lib/i18n/ko.ts
• webview-ui/src/lib/i18n/zh.ts
  • Add localized strings for the new add-worktree mode labels.

Other (build, config, docs, etc.)

• N/A

4. Type of Change

Check all that apply.

• New feature
• Bug fix
• Refactoring (no functional changes)
• Style / UI update
• i18n (localization) addition or update
• Build / configuration change
• Documentation update
• Test addition or update

5. Testing

Describe how you verified the changes.

• npm test passed
• npm run lint passed
• Manually tested via VS Code Extension Host
• Verified in both dark and light themes (if UI was changed)

6. Screenshots (optional)

Before change

After change

7. Checklist

• New message types are defined in message-bus.ts (if applicable)
• UI strings are added to both en.ts and ko.ts (if applicable)
• Extension-side strings are added to both l10n/bundle.l10n.json and bundle.l10n.ko.json (if applicable)
• No leftover console.log or debug code

8. Additional Context (optional)

The existing-branch option intentionally excludes branches already checked out in worktrees because Git does not allow the same branch to be checked out in multiple worktrees.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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