feat: add Claude Code provider support

Implements Claude Code as a new AI provider that uses the Claude Code CLI
without requiring API keys. This enables users to leverage Claude models
through their local Claude Code installation.

Key changes:
- Add complete AI SDK v1 implementation for Claude Code provider
  - Custom SDK with streaming/non-streaming support
  - Session management for conversation continuity
  - JSON extraction for object generation mode
  - Support for advanced settings (maxTurns, allowedTools, etc.)

- Integrate Claude Code into Task Master's provider system
  - Update ai-services-unified.js to handle keyless authentication
  - Add provider to supported-models.json with opus/sonnet models
  - Ensure correct maxTokens values are applied (opus: 32000, sonnet: 64000)

- Fix maxTokens configuration issue
  - Add max_tokens property to getAvailableModels() output
  - Update setModel() to properly handle claude-code models
  - Create update-config-tokens.js utility for init process

- Add comprehensive documentation
  - User guide with configuration examples
  - Advanced settings explanation and future integration options

The implementation maintains full backward compatibility with existing
providers while adding seamless Claude Code support to all Task Master
commands.

src/ai-providers/custom-sdk/claude-code/message-converter.js

@@ -0,0 +1,139 @@
+/**
+ * @fileoverview Converts AI SDK prompt format to Claude Code message format
+ */
+
+/**
+ * Convert AI SDK prompt to Claude Code messages format
+ * @param {Array} prompt - AI SDK prompt array
+ * @param {Object} [mode] - Generation mode
+ * @param {string} mode.type - Mode type ('regular', 'object-json', 'object-tool')
+ * @returns {{messagesPrompt: string, systemPrompt?: string}}
+ */
+export function convertToClaudeCodeMessages(prompt, mode) {
+	const messages = [];
+	let systemPrompt;
+
+	for (const message of prompt) {
+		switch (message.role) {
+			case 'system':
+				systemPrompt = message.content;
+				break;
+
+			case 'user':
+				if (typeof message.content === 'string') {
+					messages.push(message.content);
+				} else {
+					// Handle multi-part content
+					const textParts = message.content
+						.filter((part) => part.type === 'text')
+						.map((part) => part.text)
+						.join('\n');
+
+					if (textParts) {
+						messages.push(textParts);
+					}
+
+					// Note: Image parts are not supported by Claude Code CLI
+					const imageParts = message.content.filter(
+						(part) => part.type === 'image'
+					);
+					if (imageParts.length > 0) {
+						console.warn(
+							'Claude Code CLI does not support image inputs. Images will be ignored.'
+						);
+					}
+				}
+				break;
+
+			case 'assistant':
+				if (typeof message.content === 'string') {
+					messages.push(`Assistant: ${message.content}`);
+				} else {
+					const textParts = message.content
+						.filter((part) => part.type === 'text')
+						.map((part) => part.text)
+						.join('\n');
+
+					if (textParts) {
+						messages.push(`Assistant: ${textParts}`);
+					}
+
+					// Handle tool calls if present
+					const toolCalls = message.content.filter(
+						(part) => part.type === 'tool-call'
+					);
+					if (toolCalls.length > 0) {
+						// For now, we'll just note that tool calls were made
+						messages.push(`Assistant: [Tool calls made]`);
+					}
+				}
+				break;
+
+			case 'tool':
+				// Tool results could be included in the conversation
+				messages.push(
+					`Tool Result (${message.content[0].toolName}): ${JSON.stringify(
+						message.content[0].result
+					)}`
+				);
+				break;
+		}
+	}
+
+	// For the SDK, we need to provide a single prompt string
+	// Format the conversation history properly
+
+	// Combine system prompt with messages
+	let finalPrompt = '';
+
+	// Add system prompt at the beginning if present
+	if (systemPrompt) {
+		finalPrompt = systemPrompt;
+	}
+
+	if (messages.length === 0) {
+		return { messagesPrompt: finalPrompt, systemPrompt };
+	}
+
+	// Format messages
+	const formattedMessages = [];
+	for (let i = 0; i < messages.length; i++) {
+		const msg = messages[i];
+		// Check if this is a user or assistant message based on content
+		if (msg.startsWith('Assistant:') || msg.startsWith('Tool Result')) {
+			formattedMessages.push(msg);
+		} else {
+			// User messages
+			formattedMessages.push(`Human: ${msg}`);
+		}
+	}
+
+	// Combine system prompt with messages
+	if (finalPrompt) {
+		finalPrompt = finalPrompt + '\n\n' + formattedMessages.join('\n\n');
+	} else {
+		finalPrompt = formattedMessages.join('\n\n');
+	}
+
+	// For JSON mode, add explicit instruction to ensure JSON output
+	if (mode?.type === 'object-json') {
+		// Make the JSON instruction even more explicit
+		finalPrompt = `${finalPrompt}
+
+CRITICAL INSTRUCTION: You MUST respond with ONLY valid JSON. Follow these rules EXACTLY:
+1. Start your response with an opening brace {
+2. End your response with a closing brace }
+3. Do NOT include any text before the opening brace
+4. Do NOT include any text after the closing brace
+5. Do NOT use markdown code blocks or backticks
+6. Do NOT include explanations or commentary
+7. The ENTIRE response must be valid JSON that can be parsed with JSON.parse()
+
+Begin your response with { and end with }`;
+	}
+
+	return {
+		messagesPrompt: finalPrompt,
+		systemPrompt
+	};
+}