Fix remote proxy notification spam with cooldown, backoff, and SSH keepalive (#2325) (#2330)

- Add 5-minute per-host cooldown for remote error notifications
- Add exponential backoff (capped at 60s) to proxy broker and session controller retries
- Add default SSH ConnectTimeout/ServerAliveInterval/ServerAliveCountMax to detect dead connections faster
- Fix error status clearing to only reset on actual .connected state

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

CLI/cmux.swift

@@ -4163,6 +4163,15 @@ struct CMUXCLI {
             remoteRelayPort: options.remoteRelayPort
         )
         var parts: [String] = ["ssh"]
+        if !hasSSHOptionKey(effectiveSSHOptions, key: "ConnectTimeout") {
+            parts += ["-o", "ConnectTimeout=6"]
+        }
+        if !hasSSHOptionKey(effectiveSSHOptions, key: "ServerAliveInterval") {
+            parts += ["-o", "ServerAliveInterval=20"]
+        }
+        if !hasSSHOptionKey(effectiveSSHOptions, key: "ServerAliveCountMax") {
+            parts += ["-o", "ServerAliveCountMax=2"]
+        }
         if !hasSSHOptionKey(effectiveSSHOptions, key: "SetEnv") {
             parts += ["-o", "SetEnv COLORTERM=truecolor"]
         }