refactor: Reorganize src/ into modular folder structure

Split large monolithic files into focused modules:
- cloudcode-client.js (1,107 lines) → src/cloudcode/ (9 files)
- account-manager.js (639 lines) → src/account-manager/ (5 files)
- Move auth files to src/auth/ (oauth, token-extractor, database)
- Move CLI to src/cli/accounts.js

Update all import paths and documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/cloudcode/request-builder.js

@@ -0,0 +1,68 @@
+/**
+ * Request Builder for Cloud Code
+ *
+ * Builds request payloads and headers for the Cloud Code API.
+ */
+
+import crypto from 'crypto';
+import {
+    ANTIGRAVITY_HEADERS,
+    getModelFamily,
+    isThinkingModel
+} from '../constants.js';
+import { convertAnthropicToGoogle } from '../format/index.js';
+import { deriveSessionId } from './session-manager.js';
+
+/**
+ * Build the wrapped request body for Cloud Code API
+ *
+ * @param {Object} anthropicRequest - The Anthropic-format request
+ * @param {string} projectId - The project ID to use
+ * @returns {Object} The Cloud Code API request payload
+ */
+export function buildCloudCodeRequest(anthropicRequest, projectId) {
+    const model = anthropicRequest.model;
+    const googleRequest = convertAnthropicToGoogle(anthropicRequest);
+
+    // Use stable session ID derived from first user message for cache continuity
+    googleRequest.sessionId = deriveSessionId(anthropicRequest);
+
+    const payload = {
+        project: projectId,
+        model: model,
+        request: googleRequest,
+        userAgent: 'antigravity',
+        requestId: 'agent-' + crypto.randomUUID()
+    };
+
+    return payload;
+}
+
+/**
+ * Build headers for Cloud Code API requests
+ *
+ * @param {string} token - OAuth access token
+ * @param {string} model - Model name
+ * @param {string} accept - Accept header value (default: 'application/json')
+ * @returns {Object} Headers object
+ */
+export function buildHeaders(token, model, accept = 'application/json') {
+    const headers = {
+        'Authorization': `Bearer ${token}`,
+        'Content-Type': 'application/json',
+        ...ANTIGRAVITY_HEADERS
+    };
+
+    const modelFamily = getModelFamily(model);
+
+    // Add interleaved thinking header only for Claude thinking models
+    if (modelFamily === 'claude' && isThinkingModel(model)) {
+        headers['anthropic-beta'] = 'interleaved-thinking-2025-05-14';
+    }
+
+    if (accept !== 'application/json') {
+        headers['Accept'] = accept;
+    }
+
+    return headers;
+}