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/claude-code.js

@@ -0,0 +1,47 @@
+/**
+ * src/ai-providers/claude-code.js
+ *
+ * Implementation for interacting with Claude models via Claude Code CLI
+ * using a custom AI SDK implementation.
+ */
+
+import { createClaudeCode } from './custom-sdk/claude-code/index.js';
+import { BaseAIProvider } from './base-provider.js';
+
+export class ClaudeCodeProvider extends BaseAIProvider {
+	constructor() {
+		super();
+		this.name = 'Claude Code';
+	}
+
+	/**
+	 * Override validateAuth to skip API key validation for Claude Code
+	 * @param {object} params - Parameters to validate
+	 */
+	validateAuth(params) {
+		// Claude Code doesn't require an API key
+		// No validation needed
+	}
+
+	/**
+	 * Creates and returns a Claude Code client instance.
+	 * @param {object} params - Parameters for client initialization
+	 * @param {string} [params.baseURL] - Optional custom API endpoint (not used by Claude Code)
+	 * @returns {Function} Claude Code client function
+	 * @throws {Error} If initialization fails
+	 */
+	getClient(params) {
+		try {
+			// Claude Code doesn't use API keys or base URLs
+			// Just return the provider factory
+			return createClaudeCode({
+				defaultSettings: {
+					// Add any default settings if needed
+					// These can be overridden per request
+				}
+			});
+		} catch (error) {
+			this.handleError('client initialization', error);
+		}
+	}
+}