Implement Gemini signature caching and thinking recovery

- Add in-memory signature cache to restore thoughtSignatures stripped by Claude Code
- Implement thinking recovery logic to handle interrupted tool loops for Gemini
- Enhance schema sanitizer to preserve constraints and enums as description hints
- Update CLAUDE.md with new architecture details

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>

src/format/signature-cache.js

@@ -0,0 +1,65 @@
+/**
+ * Signature Cache
+ * In-memory cache for Gemini thoughtSignatures
+ *
+ * Gemini models require thoughtSignature on tool calls, but Claude Code
+ * strips non-standard fields. This cache stores signatures by tool_use_id
+ * so they can be restored in subsequent requests.
+ */
+
+import { GEMINI_SIGNATURE_CACHE_TTL_MS } from '../constants.js';
+
+const signatureCache = new Map();
+
+/**
+ * Store a signature for a tool_use_id
+ * @param {string} toolUseId - The tool use ID
+ * @param {string} signature - The thoughtSignature to cache
+ */
+export function cacheSignature(toolUseId, signature) {
+    if (!toolUseId || !signature) return;
+    signatureCache.set(toolUseId, {
+        signature,
+        timestamp: Date.now()
+    });
+}
+
+/**
+ * Get a cached signature for a tool_use_id
+ * @param {string} toolUseId - The tool use ID
+ * @returns {string|null} The cached signature or null if not found/expired
+ */
+export function getCachedSignature(toolUseId) {
+    if (!toolUseId) return null;
+    const entry = signatureCache.get(toolUseId);
+    if (!entry) return null;
+
+    // Check TTL
+    if (Date.now() - entry.timestamp > GEMINI_SIGNATURE_CACHE_TTL_MS) {
+        signatureCache.delete(toolUseId);
+        return null;
+    }
+
+    return entry.signature;
+}
+
+/**
+ * Clear expired entries from the cache
+ * Can be called periodically to prevent memory buildup
+ */
+export function cleanupCache() {
+    const now = Date.now();
+    for (const [key, entry] of signatureCache) {
+        if (now - entry.timestamp > GEMINI_SIGNATURE_CACHE_TTL_MS) {
+            signatureCache.delete(key);
+        }
+    }
+}
+
+/**
+ * Get the current cache size (for debugging)
+ * @returns {number} Number of entries in the cache
+ */
+export function getCacheSize() {
+    return signatureCache.size;
+}