fix(server,agent): apply Tasks 3-5 review fixes (non-blocking)

From the secure-session-core Tasks 3-5 code review (APPROVE-WITH-FIXES):
- MEDIUM-2: delete the dead `validate_agent_key` "accept-any-key" placeholder +
  its AuthenticatedAgent/AuthState scaffolding (zero callers; the real agent
  auth is validate_agent_api_key + per-agent cak_ keys). Removes an auth landmine.
- LOW-3: stop interpolating support-code values into 3 relay log lines (bearer
  credentials).
- LOW-1: document the X-Real-IP trust requirement in ip_extract.rs (NPM must set
  it from $remote_addr); behavior unchanged.
- LOW-2: correct the consent/heartbeat comment in agent session loop (the loop
  awaits the dialog; safe because CONSENT_TIMEOUT 60s < HEARTBEAT_TIMEOUT 90s).
cargo fmt/clippy(-D warnings)/test all green on GURU-5070 (89 tests, 0 warnings).
MEDIUM-1 (viewer-token logout revocation) remains a tracked follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/session/mod.rs

@@ -555,8 +555,14 @@ impl SessionManager {
             access
         );
 
-        // The MessageBox blocks the calling thread; run it on the blocking pool
-        // so the agent's async loop is not stalled and heartbeats keep flowing.
+        // The MessageBox blocks the calling thread, so it runs on the blocking
+        // pool to avoid stalling the tokio runtime. Note, however, that the main
+        // session loop `.await`s this method (see the ConsentRequest arm), so
+        // the loop is SUSPENDED for the user's entire think-time and does NOT
+        // process or respond to server heartbeats while the dialog is open.
+        // This is safe because CONSENT_TIMEOUT_SECS (60s, server-side) is within
+        // the server's 90s HEARTBEAT_TIMEOUT_SECS: the prompt resolves before the
+        // server would consider the agent dead, so the session is not torn down.
         let granted = tokio::task::spawn_blocking(move || prompt_consent(&technician_name, access))
             .await
             .unwrap_or_else(|e| {