Add query template insertion and snippet management

The plugin currently documents three query templates in the repository, but there's no built-in way to insert them directly into notes. Adding template management would streamline the workflow for users creating DQL queries.

**Proposed Changes:**
- Add a command to browse and insert available query templates from `templates/` directory
- Create a settings panel to manage custom snippets
- Support template variables (e.g., `{timeRange}`, `{dataSource}`) that can be filled interactively
- Add keyboard shortcut for quick template insertion

**Benefits:**
- Faster query writing with reusable patterns
- Reduced boilerplate for common analysis tasks
- Better discoverability of template examples

**Related Code:**
The plugin already registers commands in `src/main.ts` (e.g., `insert-dql-code-block`). Template insertion would follow a similar pattern.

Start by reading command registration in src/main.ts, especially insert-dql-code-block, then inspect the existing templates/ directory. Define completion around browsing and inserting templates, managing custom snippets, filling template variables interactively, and triggering insertion with a keyboard shortcut; the issue does not name tests or additional implementation files.

索引モデルが issue の本文から書いたものです。