merge: integrate upstream/main (v1.2.15) into feature/webui

- Resolved conflict in src/constants.js: kept config-driven approach

- Adopted upstream 10-second cooldown default

- Added MAX_EMPTY_RESPONSE_RETRIES constant from upstream

- Incorporated new test files and GitHub issue templates

src/constants.js

@@ -37,7 +37,7 @@ function getPlatformUserAgent() {
 }
 
 // Cloud Code API endpoints (in fallback order)
-const ANTIGRAVITY_ENDPOINT_DAILY = 'https://daily-cloudcode-pa.sandbox.googleapis.com';
+const ANTIGRAVITY_ENDPOINT_DAILY = 'https://daily-cloudcode-pa.googleapis.com';
 const ANTIGRAVITY_ENDPOINT_PROD = 'https://cloudcode-pa.googleapis.com';
 
 // Endpoint fallback order (daily → prod)
@@ -76,8 +76,9 @@ export const ACCOUNT_CONFIG_PATH = config?.accountConfigPath || join(
 // Uses platform-specific path detection
 export const ANTIGRAVITY_DB_PATH = getAntigravityDbPath();
 
-export const DEFAULT_COOLDOWN_MS = config?.defaultCooldownMs || (60 * 1000); // From config or 1 minute
+export const DEFAULT_COOLDOWN_MS = config?.defaultCooldownMs || (10 * 1000); // From config or 10 seconds (upstream default)
 export const MAX_RETRIES = config?.maxRetries || 5; // From config or 5
+export const MAX_EMPTY_RESPONSE_RETRIES = 2; // Max retries for empty API responses (from upstream)
 export const MAX_ACCOUNTS = config?.maxAccounts || 10; // From config or 10
 
 // Rate limit wait thresholds
@@ -146,6 +147,11 @@ export const OAUTH_CONFIG = {
 };
 export const OAUTH_REDIRECT_URI = `http://localhost:${OAUTH_CONFIG.callbackPort}/oauth-callback`;
 
+// Minimal Antigravity system instruction (from CLIProxyAPI)
+// Only includes the essential identity portion to reduce token usage and improve response quality
+// Reference: GitHub issue #76, CLIProxyAPI, gcli2api
+export const ANTIGRAVITY_SYSTEM_INSTRUCTION = `You are Antigravity, a powerful agentic AI coding assistant designed by the Google Deepmind team working on Advanced Agentic Coding.You are pair programming with a USER to solve their coding task. The task may require creating a new codebase, modifying or debugging an existing codebase, or simply answering a question.**Absolute paths only****Proactiveness**`;
+
 // Model fallback mapping - maps primary model to fallback when quota exhausted
 export const MODEL_FALLBACK_MAP = {
     'gemini-3-pro-high': 'claude-opus-4-5-thinking',
@@ -168,6 +174,7 @@ export default {
     ANTIGRAVITY_DB_PATH,
     DEFAULT_COOLDOWN_MS,
     MAX_RETRIES,
+    MAX_EMPTY_RESPONSE_RETRIES,
     MAX_ACCOUNTS,
     MAX_WAIT_BEFORE_ERROR_MS,
     MIN_SIGNATURE_LENGTH,
@@ -178,5 +185,6 @@ export default {
     isThinkingModel,
     OAUTH_CONFIG,
     OAUTH_REDIRECT_URI,
-    MODEL_FALLBACK_MAP
+    MODEL_FALLBACK_MAP,
+    ANTIGRAVITY_SYSTEM_INSTRUCTION
 };