docs: fix .cursor/ helper docs

The .cursor/ directory holds the dev-side helper docs that Cursor and
contributors using AI tooling rely on to bootstrap plugin development.
Several of them had the same bug patterns as the user-facing docs.

.cursor/plugin_templates/QUICK_START.md
- "Adding Image Rendering" section showed
  display_manager.draw_image(image, x=0, y=0). That method doesn't
  exist on DisplayManager (same bug as PLUGIN_API_REFERENCE.md and
  PLUGIN_DEVELOPMENT_GUIDE.md). Replaced with the canonical
  display_manager.image.paste((x,y)) pattern, including the
  transparency-mask form.

.cursor/plugins_guide.md
- 10 occurrences of ./dev_plugin_setup.sh — the script lives at
  scripts/dev/dev_plugin_setup.sh, so anyone copy-pasting these
  examples gets "command not found". Bulk fixed via sed.
- "Test with emulator: python run.py --emulator" — there's no
  --emulator flag. Replaced with the real options:
  EMULATOR=true python3 run.py for the full display, or
  scripts/dev_server.py for the dev preview.
- Secrets management section showed a fictional
  "config_secrets": { "api_key": "my-plugin.api_key" } reference
  field. Verified in src/config_manager.py:162-172 that secrets are
  loaded by deep-merging config_secrets.json into the main config.
  There is no separate reference field — just put the secret under
  the same plugin namespace and read it from the merged config.
  Rewrote the section with the real pattern.
- "ssh pi@raspberrypi" -> "ssh ledpi@your-pi-ip" (consistent with
  the rest of LEDMatrix docs which use ledpi as the default user)

.cursor/README.md
- Same ./dev_plugin_setup.sh -> ./scripts/dev/dev_plugin_setup.sh
  fix (×6 occurrences via replace_all).
- Same "python run.py --emulator" -> "EMULATOR=true python3 run.py"
  fix. Also added a pointer to scripts/dev_server.py for previewing
  plugins without running the full display.
- "Example Plugins: plugins/hockey-scoreboard/" — the canonical
  source is the ledmatrix-plugins repo. Installed copies land in
  plugin-repos/ or plugins/. Updated the line to point at the
  ledmatrix-plugins repo and explain both local locations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.cursor/plugin_templates/QUICK_START.md

@@ -156,20 +156,34 @@ def _fetch_data(self):
 
 ### Adding Image Rendering
 
+There is no `draw_image()` helper on `DisplayManager`. To render an
+image, paste it directly onto the underlying PIL `Image`
+(`display_manager.image`) and then call `update_display()`:
+
 ```python
 def _render_content(self):
-    # Load and render image
-    image = Image.open("assets/logo.png")
-    self.display_manager.draw_image(image, x=0, y=0)
-    
+    # Load and paste image onto the display canvas
+    image = Image.open("assets/logo.png").convert("RGB")
+    self.display_manager.image.paste(image, (0, 0))
+
     # Draw text overlay
     self.display_manager.draw_text(
         "Text",
         x=10, y=20,
         color=(255, 255, 255)
     )
+
+    self.display_manager.update_display()
 ```
 
+For transparency, paste with a mask:
+
+```python
+icon = Image.open("assets/icon.png").convert("RGBA")
+self.display_manager.image.paste(icon, (5, 5), icon)
+```
+
+
 ### Adding Live Priority
 
 1. Enable in config: