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/credential_store.py

@@ -0,0 +1,69 @@
+"""Echo Core secrets manager — keyring wrapper."""
+
+import json
+import keyring
+
+SERVICE = "echo-core"
+
+# Required secrets that should exist for full functionality
+REQUIRED_SECRETS = ["discord_token"]
+
+
+def set_secret(name: str, value: str) -> None:
+    """Store a secret in the system keyring."""
+    keyring.set_password(SERVICE, name, value)
+    # Track secret name in the registry
+    names = _get_registry()
+    if name not in names:
+        names.append(name)
+        _save_registry(names)
+
+
+def get_secret(name: str) -> str | None:
+    """Retrieve a secret from keyring. Returns None if not found."""
+    return keyring.get_password(SERVICE, name)
+
+
+def get_json_secret(name: str) -> dict | None:
+    """Retrieve and JSON-deserialize a secret."""
+    raw = get_secret(name)
+    if raw is None:
+        return None
+    return json.loads(raw)
+
+
+def delete_secret(name: str) -> bool:
+    """Delete a secret. Returns True if existed."""
+    try:
+        keyring.delete_password(SERVICE, name)
+    except keyring.errors.PasswordDeleteError:
+        pass
+    names = _get_registry()
+    if name in names:
+        names.remove(name)
+        _save_registry(names)
+        return True
+    return False
+
+
+def list_secrets() -> list[str]:
+    """List all stored secret names."""
+    return _get_registry()
+
+
+def check_secrets() -> dict[str, bool]:
+    """Check which required secrets exist. Returns {name: exists}."""
+    return {name: get_secret(name) is not None for name in REQUIRED_SECRETS}
+
+
+def _get_registry() -> list[str]:
+    """Get list of secret names from keyring registry."""
+    raw = keyring.get_password(SERVICE, "_registry")
+    if raw is None:
+        return []
+    return json.loads(raw)
+
+
+def _save_registry(names: list[str]) -> None:
+    """Save secret names registry to keyring."""
+    keyring.set_password(SERVICE, "_registry", json.dumps(sorted(set(names))))