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/json-extractor.js

@@ -0,0 +1,59 @@
+/**
+ * @fileoverview Extract JSON from Claude's response, handling markdown blocks and other formatting
+ */
+
+/**
+ * Extract JSON from Claude's response
+ * @param {string} text - The text to extract JSON from
+ * @returns {string} - The extracted JSON string
+ */
+export function extractJson(text) {
+	// Remove markdown code blocks if present
+	let jsonText = text.trim();
+
+	// Remove ```json blocks
+	jsonText = jsonText.replace(/^```json\s*/gm, '');
+	jsonText = jsonText.replace(/^```\s*/gm, '');
+	jsonText = jsonText.replace(/```\s*$/gm, '');
+
+	// Remove common TypeScript/JavaScript patterns
+	jsonText = jsonText.replace(/^const\s+\w+\s*=\s*/, ''); // Remove "const varName = "
+	jsonText = jsonText.replace(/^let\s+\w+\s*=\s*/, ''); // Remove "let varName = "
+	jsonText = jsonText.replace(/^var\s+\w+\s*=\s*/, ''); // Remove "var varName = "
+	jsonText = jsonText.replace(/;?\s*$/, ''); // Remove trailing semicolons
+
+	// Try to extract JSON object or array
+	const objectMatch = jsonText.match(/{[\s\S]*}/);
+	const arrayMatch = jsonText.match(/\[[\s\S]*\]/);
+
+	if (objectMatch) {
+		jsonText = objectMatch[0];
+	} else if (arrayMatch) {
+		jsonText = arrayMatch[0];
+	}
+
+	// First try to parse as valid JSON
+	try {
+		JSON.parse(jsonText);
+		return jsonText;
+	} catch {
+		// If it's not valid JSON, it might be a JavaScript object literal
+		// Try to convert it to valid JSON
+		try {
+			// This is a simple conversion that handles basic cases
+			// Replace unquoted keys with quoted keys
+			const converted = jsonText
+				.replace(/([{,]\s*)([a-zA-Z_$][a-zA-Z0-9_$]*)\s*:/g, '$1"$2":')
+				// Replace single quotes with double quotes
+				.replace(/'/g, '"');
+
+			// Validate the converted JSON
+			JSON.parse(converted);
+			return converted;
+		} catch {
+			// If all else fails, return the original text
+			// The AI SDK will handle the error appropriately
+			return text;
+		}
+	}
+}