feat: add native Pi package support

prompts/export-diagram.md

@@ -0,0 +1,30 @@
+---
+description: Export a diagram-design HTML file to SVG and PNG
+argument-hint: "<html-file> [--svg-only|--png-only] [--scale=N] [--output=<path>]"
+---
+
+Export diagram HTML at `$1` to `.svg` or `.png`, following `skills/diagram-design/references/export.md` from installed diagram-design package. Read that reference first; treat it as source of truth.
+
+Full argument string: `$ARGUMENTS`
+
+## Defaults
+
+- Produce both `.svg` and `.png` next to source (for example, `diagram.html` → `diagram.svg` + `diagram.png`).
+- Render PNG at `device_scale_factor=2`.
+
+## Flags
+
+- `--svg-only` — emit SVG only. Skip Playwright.
+- `--png-only` — emit PNG only.
+- `--scale=1`, `--scale=2`, or `--scale=3` — override PNG device scale factor.
+- `--output=<path>` — override output base path; append format extension.
+
+## Required behavior
+
+1. No source path → ask user which `.html` file to export. Do not guess.
+2. Source is `assets/index.html` → refuse; ask for specific diagram file.
+3. Source lacks `<svg>` → refuse; write nothing.
+4. PNG requested without Playwright → show install instruction from reference verbatim; stop. Do not auto-install.
+5. `--scale` outside {1, 2, 3} → reject.
+
+After export, report output paths plus sizes.