[no bump] Move technical docs to docs/, add icon and PNG export tool

README now carries the icon, TL;DR, description and features; the
technical sections live in docs/. tools/icon_png.py renders
images/icon.svg to a square PNG (512x512 by default, --resolution N).
plugin.json points at the plugin's own icon.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/icon_png.py

@@ -0,0 +1,96 @@
+#!/usr/bin/env python3
+"""Renders images/icon.svg to a square PNG (512x512 by default).
+
+Stdlib only: the actual rasterising is delegated to the first renderer found,
+in this order: the cairosvg Python package, rsvg-convert (librsvg),
+ImageMagick (magick/convert), Inkscape. Install any one of them.
+
+Usage:
+    tools/icon_png.py                         # images/icon.svg -> images/icon.png, 512x512
+    tools/icon_png.py --resolution 1024       # 1024x1024
+    tools/icon_png.py other.svg -o out.png    # explicit input/output
+
+The output is always PNG, always <resolution>x<resolution>.
+"""
+from __future__ import annotations
+
+import argparse
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+from typing import List, Optional
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+DEFAULT_SVG = REPO_ROOT / "images" / "icon.svg"
+DEFAULT_RESOLUTION = 512
+
+
+def fail(message: str) -> "None":
+    print(f"error: {message}", file=sys.stderr)
+    raise SystemExit(1)
+
+
+def cli_command(svg: Path, png: Path, size: int) -> Optional[List[str]]:
+    """Returns the command line for the first installed CLI renderer, if any."""
+    if shutil.which("rsvg-convert"):
+        return ["rsvg-convert", "-w", str(size), "-h", str(size), "-f", "png",
+                "-o", str(png), str(svg)]
+    magick = shutil.which("magick") or shutil.which("convert")
+    if magick:
+        # A high density rasterises the vector sharply before scaling down;
+        # "!" forces the exact size.
+        return [magick, "-background", "none", "-density", "1200", str(svg),
+                "-resize", f"{size}x{size}!", f"png:{png}"]
+    if shutil.which("inkscape"):
+        return ["inkscape", str(svg), "--export-type=png", f"--export-filename={png}",
+                "-w", str(size), "-h", str(size)]
+    return None
+
+
+def render(svg: Path, png: Path, size: int) -> str:
+    """Renders svg into png at size x size, returning the renderer's name."""
+    try:
+        import cairosvg  # type: ignore[import-not-found]
+    except ImportError:
+        pass
+    else:
+        cairosvg.svg2png(url=str(svg), write_to=str(png),
+                         output_width=size, output_height=size)
+        return "cairosvg"
+
+    command = cli_command(svg, png, size)
+    if command is None:
+        fail("no SVG renderer found; install one of: cairosvg (pip), "
+             "rsvg-convert (librsvg), ImageMagick, Inkscape")
+    result = subprocess.run(command, capture_output=True, text=True)
+    if result.returncode != 0:
+        fail(f"{Path(command[0]).name} failed:\n{result.stderr.strip()}")
+    return Path(command[0]).name
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description="Render an SVG icon to a square PNG.")
+    parser.add_argument("svg", nargs="?", type=Path, default=DEFAULT_SVG,
+                        help="input SVG (default: images/icon.svg)")
+    parser.add_argument("-r", "--resolution", type=int, default=DEFAULT_RESOLUTION,
+                        help=f"width and height in pixels (default: {DEFAULT_RESOLUTION})")
+    parser.add_argument("-o", "--output", type=Path,
+                        help="output PNG (default: the input path with a .png suffix)")
+    args = parser.parse_args()
+
+    if args.resolution <= 0:
+        fail("--resolution must be a positive integer")
+    if not args.svg.is_file():
+        fail(f"{args.svg} not found")
+    png = args.output or args.svg.with_suffix(".png")
+    if png.suffix.lower() != ".png":
+        fail(f"output must be a .png file, got {png}")
+    png.parent.mkdir(parents=True, exist_ok=True)
+
+    renderer = render(args.svg, png, args.resolution)
+    print(f"{png} ({args.resolution}x{args.resolution}, via {renderer})")
+
+
+if __name__ == "__main__":
+    main()