rename secrets.py to credential_store.py, enhance /status, add usage tracking

- Rename src/secrets.py → src/credential_store.py (avoid stdlib conflict)
- Enhanced /status command: uptime, tokens, cost, context window usage
- Session metadata now tracks input/output tokens, cost, duration
- _safe_env() changed from allowlist to blocklist approach
- Better Claude CLI error logging

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.py

@@ -7,12 +7,16 @@ import signal
 import sys
 from pathlib import Path
 
+# Ensure project root is on sys.path so `src.*` imports work
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+if str(PROJECT_ROOT) not in sys.path:
+    sys.path.insert(0, str(PROJECT_ROOT))
+
 from src.config import load_config
-from src.secrets import get_secret
+from src.credential_store import get_secret
 from src.adapters.discord_bot import create_bot, split_message
 from src.scheduler import Scheduler
 
-PROJECT_ROOT = Path(__file__).resolve().parent.parent
 PID_FILE = PROJECT_ROOT / "echo-core.pid"
 LOG_DIR = PROJECT_ROOT / "logs"