Add warming doc

docs/warming.md

@@ -0,0 +1,15 @@
+# Warming (OpenCode V2)
+
+Session warming sends periodic model requests to preserve provider-side prompt caches or short-lived state while you pause between prompts. Disabled by default.
+
+Enable in config:
+{
+  "$schema": "https://opencode.ai/config.json",
+  "warming": true
+}
+
+Options: prompt, interval, duration. interval can be "auto" using 90% of model cache.ttl.
+
+Timing: starts after non-warming request; warming requests at interval; stops after duration. New activity resets timers.
+
+Failures logged without failing session. Costs are real provider requests.