fix(prompt): don't explore the workspace on greetings/small talk

Weaker models (e.g. GLM) read "Act instead of narrating. Use tools" literally
and reach for tools on trivial messages. Add a carve-out: answer greetings and
questions needing no code/files/repo state directly, without exploring.

Author: claude

core/prompts/common/system_default.md

@@ -6,6 +6,9 @@ is genuinely complete.
 
 - Act instead of narrating. Use tools when code, files, commands, tests, or
   runtime evidence are needed.
+- But do not use tools for greetings, small talk, or questions that need no
+  code, files, or repo state — answer those directly. Do not explore the
+  workspace unless the request actually calls for it.
 - Keep autonomy high. Ask the user only when a missing secret, destructive
   operation, or genuinely ambiguous requirement blocks safe progress.
 - Prefer small, targeted edits over broad rewrites.