docs(rules): add plugin-output-styles rule to prevent directory paths

Documents that outputStyles must use explicit file paths, not
directory paths like "./styles/". Includes examples of correct
vs incorrect format and explains the validation error.

Prevents recurrence of the bug fixed in 82e0eaa.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: aRustyDev

.claude/rules/plugin-output-styles.md

@@ -0,0 +1,55 @@
+# Plugin outputStyles Format
+
+When defining `outputStyles` in a plugin's `plugin.json`, use explicit file paths only.
+
+## Rule
+
+**DO NOT** use directory paths in the `outputStyles` array:
+
+```json
+// WRONG - causes validation errors
+"outputStyles": [
+  "./styles/",
+  "../../output-styles/feedback-submission.md"
+]
+```
+
+**DO** use explicit file paths:
+
+```json
+// CORRECT
+"outputStyles": [
+  "./styles/swift-code.md",
+  "./styles/swiftui-view.md",
+  "../../output-styles/feedback-submission.md"
+]
+```
+
+## Why
+
+Directory paths like `"./styles/"` cause plugin validation errors:
+
+```text
+Plugin has an invalid manifest file. Validation errors: outputStyles: Invalid input
+```
+
+The validator expects each entry to be a path to a specific `.md` file.
+
+## When Scaffolding Plugins
+
+1. If the plugin has custom styles in `./styles/`, list each `.md` file explicitly
+2. If no custom styles exist, only include the shared feedback style:
+
+   ```json
+   "outputStyles": ["../../output-styles/feedback-submission.md"]
+   ```
+
+3. Never copy `outputStyles` from existing plugins without verifying the format
+
+## Template Reference
+
+The correct template format is in `context/plugins/.template/.claude-plugin/plugin.json`:
+
+```json
+"outputStyles": ["../../output-styles/feedback-submission.md"]
+```