🔧 fix(vscode): worktree 目录配置改用插件自带设置面板，撤销 VS Code 原生配置

上一版误把 worktree 目录模板注册成 VS Code 原生设置（contributes.configuration
+ workspace.getConfiguration），但本插件有自己的设置面板（SettingsModal，存 globalState）。
改为：
- Settings 新增 worktreeDirectory 字段（默认 ~/Sources/worktree/$project_name/$feature_name，
  留空回退默认），贯穿 store/handler/两端 messages/useIssues/App/SettingsModal
- SettingsModal「钩子」分组新增「工作树目录」输入项，走既有 save/load 流程
- handleImplement 改从 getSettings(panel.context).worktreeDirectory 读模板
- 删除 package.json 的 contributes.configuration（meta.ts 为 gitignore 生成物，已本地还原）

deriveSlug / resolveWorktreeDir / ~ 展开 / 绝对路径 / feature/<slug> 逻辑不变。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vscode/src/panel/KanbanPanel.ts

@@ -840,6 +840,7 @@ export class KanbanWebviewPanel {
         reviewPrompt: s.reviewPrompt,
         devBranch: s.devBranch,
         autoBuildBranch: s.autoBuildBranch,
+        worktreeDirectory: s.worktreeDirectory,
         worktreePostCreateScript: s.worktreePostCreateScript,
         worktreePreRemoveScript: s.worktreePreRemoveScript,
         implTabPreCreateScript: s.implTabPreCreateScript,
@@ -899,6 +900,7 @@ export class KanbanWebviewPanel {
           reviewPrompt: s.reviewPrompt,
           devBranch: s.devBranch,
           autoBuildBranch: s.autoBuildBranch,
+          worktreeDirectory: s.worktreeDirectory,
           worktreePostCreateScript: s.worktreePostCreateScript,
           worktreePreRemoveScript: s.worktreePreRemoveScript,
           implTabPreCreateScript: s.implTabPreCreateScript,