chat mode: json_output toggle to return clean extracted JSON

For JSON-producing system prompts (e.g. LTX prompt-relay), json_output=true pulls
the JSON object out of the reply (strips reasoning/prose/code-fences via _parse_json,
which handles nested schemas and reasoning-then-JSON) and returns it re-serialized;
falls back to raw text if none parses. agent_bridge gains --json-output.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -58,6 +58,9 @@ node — feed an image (and optionally a second), write your own `system_prompt`
 and read the model's text from the `analysis` output. Reuses the same model dropdown, quant,
 and auto-download as the judge, so it's a one-node abliterated VLM for captioning, tagging,
 Q&A, prompt-from-image, etc. (CLI: `agent_bridge.py --mode chat --user-prompt "..."`).
+Set **`json_output=true`** for JSON-producing system prompts — it extracts the JSON object
+from the reply (stripping any reasoning, prose, or ```fences) and returns it clean and
+re-serialized (falls back to raw text if none parses). Works even with `enable_thinking` on.
 
 ## Performance / speed