feat: add json log format option

Add log_format config field ("text" default, "json" for structured
JSON output) to support machine-readable logging.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

oubliette.toml.example

@@ -1,4 +1,5 @@
 log_level = "info"
+log_format = "text" # "text" or "json"
 
 [ssh]
 listen_addr = ":2222"