Add AI writing assistant plugin with local backend, brave search, and image generation support

- Implement local backend AI provider with Ollama integration
- Add Brave Search API integration for real-time search suggestions
- Add image generation manager with multiple AI providers
- Create hybrid provider system with local/cloud fallback
- Add comprehensive settings UI with provider management
- Implement Gutenberg sidebar with writing assistance controls
- Add SEO schema generation for AI-generated content
- Multiple provider support: OpenRouter, local backend, Codex

includes/interface-ai-provider.php

@@ -0,0 +1,67 @@
+<?php
+/**
+ * AI Provider Interface
+ *
+ * Common contract for all AI providers (OpenRouter, Local Backend, Codex, etc.)
+ *
+ * @package WP_Agentic_Writer
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+interface WP_Agentic_Writer_AI_Provider_Interface {
+	/**
+	 * Non-streaming chat completion
+	 *
+	 * @param array  $messages Array of message objects with 'role' and 'content'.
+	 * @param array  $options  Optional parameters (temperature, max_tokens, etc.).
+	 * @param string $type     Task type (chat, clarity, planning, writing, refinement).
+	 * @return array|WP_Error Response with content, model, tokens, cost, or error.
+	 */
+	public function chat( $messages, $options = array(), $type = 'planning' );
+
+	/**
+	 * Streaming chat completion
+	 *
+	 * @param array    $messages Array of message objects.
+	 * @param array    $options  Optional parameters.
+	 * @param string   $type     Task type.
+	 * @param callable $callback Function to call with each chunk (chunk, is_complete, accumulated).
+	 * @return array|WP_Error Response with accumulated content, tokens, cost, or error.
+	 */
+	public function chat_stream( $messages, $options = array(), $type = 'planning', $callback = null );
+
+	/**
+	 * Generate image
+	 *
+	 * @param string $prompt Image generation prompt.
+	 * @param string $model  Model to use (optional, uses provider default).
+	 * @param array  $options Optional parameters (size, quality, n).
+	 * @return array|WP_Error Response with url, model, cost, or error.
+	 */
+	public function generate_image( $prompt, $model = null, $options = array() );
+
+	/**
+	 * Check if provider is properly configured
+	 *
+	 * @return bool True if configured and ready to use.
+	 */
+	public function is_configured();
+
+	/**
+	 * Test connection to provider
+	 *
+	 * @return array|WP_Error Success array or error details.
+	 */
+	public function test_connection();
+
+	/**
+	 * Check if provider supports a specific task type
+	 *
+	 * @param string $type Task type (chat, clarity, planning, writing, refinement, image).
+	 * @return bool True if supported.
+	 */
+	public function supports_task_type( $type );
+}