docs: add Copilot instructions

.github/copilot-instructions.md

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+# Python Template Snippets
+
+## Project structure
+
+This is a declarative VS Code snippets extension, not a Python application. `package.json` registers all three snippet catalogs for the `html` language scope:
+
+- `snippets/snippets.json`: syntax shared by Jinja2 and Django.
+- `snippets/jinja-snippets.json`: Jinja2-only syntax.
+- `snippets/django-snippets.json`: Django-only syntax.
+
+The extension has no activation code. Changes to a snippet catalog take effect through the `contributes.snippets` entries in `package.json`.
+
+## Snippet conventions
+
+- Keep shared template syntax in `snippets/snippets.json`; add engine-specific syntax only to its matching Jinja or Django catalog.
+- Snippet prefixes are intentionally namespaced: shared snippets use `pt`, Jinja snippets use `ptj-`, and Django snippets use `ptd-`.
+- Each entry is keyed by a readable display name and defines `scope`, `prefix`, `body`, and `description`. All current snippets use `"scope": "html"`.
+- Write `body` as an array of lines. Preserve VS Code snippet placeholders: numbered placeholders control tab order, repeated placeholder numbers mirror the same value, and `$0` is the final cursor position. Use `\t` for indentation and explicit `"\n"` elements for blank lines, following the existing catalog style.
+- Update the README snippet reference when adding, removing, renaming, or changing a user-visible prefix or generated output.
+
+## Commands
+
+There are no npm scripts, automated tests, or lint configuration. There is consequently no single-test command.
+
+```bash
+# Install the locked packaging dependency
+npm ci
+
+# Validate all snippet catalogs parse as JSON
+node -e "for (const f of ['snippets/snippets.json','snippets/jinja-snippets.json','snippets/django-snippets.json']) JSON.parse(require('fs').readFileSync(f, 'utf8'))"
+
+# Build a VSIX package (also validates the extension manifest)
+npx vsce package
+```
+
+`npx vsce package` creates a `.vsix` file in the repository root; it is ignored by Git. The release workflow runs this same packaging command with Node.js 20.
+
+## Releases
+
+Releases are triggered by pushing a `v*` tag. Before creating a release tag, keep the tag version exactly equal to `package.json`'s `version`; the workflow fails when they differ. It then packages and publishes the extension to the VS Code Marketplace and attaches the generated VSIX to the GitHub release.
+
+Agents must not create releases or push tags. Both actions are maintainer-only responsibilities.
+
+## Commit and pull request titles
+
+Use Conventional Commits strictly for every commit message and pull request title (for example, `feat: add Django cycle snippet` or `fix: correct Jinja static URL placeholder`).