fix: preserve CLI session IDs for session resume

- Add resumeArgs to DEFAULT_CLAUDE_BACKEND for proper --resume flag usage
- Fix gateway not preserving cliSessionIds/claudeCliSessionId in nextEntry
- Add test for CLI session ID preservation in gateway agent handler
- Update docs with new resumeArgs default

src/agents/cli-backends.ts

@@ -28,6 +28,14 @@ const CLAUDE_MODEL_ALIASES: Record<string, string> = {
 const DEFAULT_CLAUDE_BACKEND: CliBackendConfig = {
   command: "claude",
   args: ["-p", "--output-format", "json", "--dangerously-skip-permissions"],
+  resumeArgs: [
+    "-p",
+    "--output-format",
+    "json",
+    "--dangerously-skip-permissions",
+    "--resume",
+    "{sessionId}",
+  ],
   output: "json",
   input: "arg",
   modelArg: "--model",