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

docs/gateway/cli-backends.md

@@ -182,6 +182,7 @@ Clawdbot ships a default for `claude-cli`:
 
 - `command: "claude"`
 - `args: ["-p", "--output-format", "json", "--dangerously-skip-permissions"]`
+- `resumeArgs: ["-p", "--output-format", "json", "--dangerously-skip-permissions", "--resume", "{sessionId}"]`
 - `modelArg: "--model"`
 - `systemPromptArg: "--append-system-prompt"`
 - `sessionArg: "--session-id"`