Add comprehensive validation rules documentation

VALIDATION_RULES.md

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+# Plugin Validation Rules
+
+All plugin submissions are automatically validated before being merged. PRs must pass all validation checks to be auto-merged.
+
+## Security
+
+**The validation script runs from the main branch, not from your PR branch.** This prevents malicious PRs from modifying the validation logic. PRs that attempt to modify `.gitea/` directory files will be automatically rejected.
+
+## File Requirements
+
+### 1. Location
+- ✅ Only modify files in `plugins/` directory
+- ❌ Cannot modify files outside `plugins/`
+- ❌ Cannot modify `.gitea/` infrastructure files
+- ⚠️ `plugins/index.json` is auto-generated - don't manually edit it
+
+### 2. File Type
+- ✅ Only `.json` files are allowed in `plugins/` directory
+- ❌ No other file types (images, scripts, etc. must be in plugin repo)
+
+## Plugin JSON Schema
+
+### Required Fields
+Every plugin JSON must have these fields:
+
+```json
+{
+  "id": "string",
+  "name": "string",
+  "version": "string",
+  "description": "string",
+  "author": "string",
+  "repository": "string"
+}
+```
+
+### Field Validation Rules
+
+#### `id` (required)
+- Must match the filename
+- Example: `example-plugin.json` → `"id": "example-plugin"`
+- Lowercase, hyphens allowed, no spaces
+
+#### `name` (required)
+- Display name for the plugin
+- Human-readable, any format
+
+#### `version` (required)
+- Must follow semantic versioning: `x.y.z`
+- Example: `"1.0.0"`, `"2.1.3"`
+- ❌ Invalid: `"v1.0"`, `"1.0"`, `"latest"`
+
+#### `description` (required)
+- Short description of what the plugin does
+- Will be shown in the plugin browser
+
+#### `author` (required)
+- **MUST MATCH YOUR GITEA USERNAME**
+- This enforces ownership - you can only submit/remove plugins authored by you
+- Case-sensitive
+
+#### `repository` (required)
+- Git repository URL where the plugin code lives
+- Must be a valid URL
+- Example: `"http://synbox.ruv.wtf:8418/username/Plugin-Name.git"`
+
+#### `thumbnail` (optional)
+- URL to plugin thumbnail image
+- **Requirements:**
+  - Format: PNG, JPG, or GIF only
+  - Max dimensions: 512×512 pixels
+  - Max file size: 2MB
+  - Must be accessible via HTTP/HTTPS
+- Example: `"http://synbox.ruv.wtf:8418/username/Plugin-Name/raw/branch/main/thumbnail.png"`
+- Validation checks actual image format (magic bytes), dimensions, and size
+
+### Optional Fields
+You can include any additional fields for metadata:
+- `homepage` - Project homepage URL
+- `downloadUrl` - Direct download link
+- `tags` - Array of tags
+- `addedDate` - ISO timestamp
+
+## Ownership Rules
+
+### Adding Plugins
+- The `author` field must match your Gitea username
+- This is checked automatically
+
+### Removing Plugins
+- You can only remove plugins where `author` matches your username
+- Attempting to remove someone else's plugin will fail validation
+
+## Validation Process
+
+1. **File check**: Ensures only `plugins/*.json` files are modified
+2. **Infrastructure check**: Blocks any `.gitea/` modifications
+3. **JSON parsing**: Validates JSON syntax
+4. **Schema validation**: Checks all required fields exist
+5. **Field validation**: Validates each field's format and rules
+6. **Thumbnail validation** (if provided): Downloads and validates image
+7. **Author matching**: Ensures `author` field matches PR creator
+8. **Auto-merge**: If all checks pass, PR is automatically merged
+
+## Testing Locally
+
+You can test your plugin JSON before submitting:
+
+```bash
+cd Plugin-Directory
+node .gitea/scripts/validate-pr.js "your-username" "plugins/your-plugin.json"
+```
+
+## Common Errors
+
+### ❌ Author mismatch
+```
+❌ plugins/example-plugin.json: Author "someone" must match PR creator "you"
+```
+**Fix**: Change `"author": "someone"` to `"author": "you"`
+
+### ❌ ID doesn't match filename
+```
+❌ plugins/my-plugin.json: Plugin ID "different-name" doesn't match filename "my-plugin.json"
+```
+**Fix**: Change `"id": "different-name"` to `"id": "my-plugin"`
+
+### ❌ Invalid version
+```
+❌ plugins/example-plugin.json: Invalid version format: v1.0
+```
+**Fix**: Use semantic versioning like `"version": "1.0.0"`
+
+### ❌ Thumbnail too large
+```
+❌ plugins/example-plugin.json: Thumbnail validation failed: Thumbnail exceeds 2MB limit (3.45MB)
+```
+**Fix**: Compress your thumbnail to under 2MB
+
+### ❌ Thumbnail dimensions too large
+```
+❌ plugins/example-plugin.json: Thumbnail validation failed: Thumbnail dimensions 1024x768 exceed 512x512
+```
+**Fix**: Resize your thumbnail to 512×512 or smaller
+
+## Example Valid Plugin JSON
+
+```json
+{
+  "id": "example-plugin",
+  "name": "Example Plugin",
+  "version": "1.0.0",
+  "description": "An example plugin demonstrating the plugin system capabilities",
+  "author": "litruv",
+  "repository": "http://synbox.ruv.wtf:8418/litruv/Plugin-Example.git",
+  "thumbnail": "http://synbox.ruv.wtf:8418/litruv/Plugin-Example/raw/branch/main/thumbnail.png",
+  "homepage": "http://synbox.ruv.wtf:8418/litruv/Plugin-Example",
+  "tags": ["example", "demo"]
+}
+```
+
+## Security Note
+
+**Never include sensitive information in plugin JSON files.** These files are public and will be served to all plugin host users. Do not include:
+- API keys or tokens
+- Passwords
+- Private URLs or endpoints
+- Personal information