feat: support reusable Nunjucks templates via vault .njk files

[victoraraujo105]

Problem

The TEMPLATE renderer currently requires the full template to be written inline in every code block. Users who have multiple queries with similar table layouts (e.g. task lists with external/internal links, status columns) must duplicate the entire template in each block. This leads to:

• Significant boilerplate in notes with multiple queries
• Difficult maintenance when the template design changes (must update every copy)
• No way to share templates across notes

Proposed Solution

Allow the TEMPLATE renderer to resolve {% include %} and {% from ... import %} directives against .njk files stored in the Obsidian vault.

Usage examples

Include a full template:

```sqlseal
TEMPLATE
{% include "_templates/task-table.njk" %}

SELECT task_id, title, state FROM files WHERE ...
```

Import macros for composable templates:

```sqlseal
TEMPLATE
{% from "_templates/macros.njk" import extLink, intLink %}
<table>
{% for row in data %}
<tr>
  <td>{{ extLink(row.url, row.id) }}</td>
  <td>{{ intLink(row.title) }}</td>
</tr>
{% endfor %}
</table>

SELECT ... FROM files WHERE ...
```

Implementation

This feature is implemented across two stacked PRs:

1. feat: replace Handlebars with Nunjucks in TEMPLATE renderer #198 — Replace Handlebars with Nunjucks in TEMPLATE renderer (prerequisite)
2. feat: add VaultLoader for reusable Nunjucks templates in TEMPLATE renderer #199 — Add VaultLoader for reusable Nunjucks templates via .njk vault files