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/session-manager.js

@@ -0,0 +1,47 @@
+/**
+ * Session Management for Cloud Code
+ *
+ * Handles session ID derivation for prompt caching continuity.
+ * Session IDs are derived from the first user message to ensure
+ * the same conversation uses the same session across turns.
+ */
+
+import crypto from 'crypto';
+
+/**
+ * Derive a stable session ID from the first user message in the conversation.
+ * This ensures the same conversation uses the same session ID across turns,
+ * enabling prompt caching (cache is scoped to session + organization).
+ *
+ * @param {Object} anthropicRequest - The Anthropic-format request
+ * @returns {string} A stable session ID (32 hex characters) or random UUID if no user message
+ */
+export function deriveSessionId(anthropicRequest) {
+    const messages = anthropicRequest.messages || [];
+
+    // Find the first user message
+    for (const msg of messages) {
+        if (msg.role === 'user') {
+            let content = '';
+
+            if (typeof msg.content === 'string') {
+                content = msg.content;
+            } else if (Array.isArray(msg.content)) {
+                // Extract text from content blocks
+                content = msg.content
+                    .filter(block => block.type === 'text' && block.text)
+                    .map(block => block.text)
+                    .join('\n');
+            }
+
+            if (content) {
+                // Hash the content with SHA256, return first 32 hex chars
+                const hash = crypto.createHash('sha256').update(content).digest('hex');
+                return hash.substring(0, 32);
+            }
+        }
+    }
+
+    // Fallback to random UUID if no user message found
+    return crypto.randomUUID();
+}