Two security hardening changes for the API server:
1. **Startup warning when no API key is configured.**
When `API_SERVER_KEY` is not set, all endpoints accept unauthenticated
requests. This is the default configuration, but operators may not
realize the security implications. A prominent warning at startup
makes the risk visible.
2. **Require authentication for session continuation.**
The `X-Hermes-Session-Id` header allows callers to load and continue
any session stored in state.db. Without authentication, an attacker
who can reach the API server (e.g. via CORS from a malicious page,
or on a shared host) could enumerate session IDs and read conversation
history — which may contain API keys, passwords, code, or other
sensitive data shared with the agent.
Session continuation now returns 403 when no API key is configured,
with a clear error message explaining how to enable the feature.
When a key IS configured, the existing Bearer token check already
gates access.
This is defense-in-depth: the API server is intended for local use,
but defense against cross-origin and shared-host attacks is important
since the default binding is 127.0.0.1 which is reachable from
browsers via DNS rebinding or localhost CORS.
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