Add native watched-folder manager

[wrgrant]

What changed

• adds a native Git Fleet: Watched Folders manager
• uses the platform folder chooser to add one or more search roots
• lists every configured extra folder with a per-folder remove button
• keeps an advanced button for editing the underlying machine-local setting
• points the navigator settings action at the new manager
• updates localization, README, changelog, and release version to 0.2.1

Why

VS Code extension settings cannot replace an array editor with a custom folder-picker widget. The manager composes native Quick Pick and folder-dialog controls around the existing portable setting, so normal use no longer requires typing filesystem paths.

Validation

• pnpm run format
• pnpm run lint
• pnpm run typecheck
• pnpm run test (90 backend, 16 extension, 43 webview)
• pnpm run package
• localization coverage: 129/129 for Simplified and Traditional Chinese
• packaged and installed wgrant-dev.git-fleet@0.2.1 locally

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.