docs(agents): spawned subagents never commit; coordinator commits once

A subagent mid-dedash sweep read the 'commit and push when reasonable'
rule and pushed its own subset before the coordinator verified the
aggregate. State the assembly rule explicitly so spawned tasks report
instead.

AGENTS.md

@@ -25,6 +25,11 @@ The workflow:
 5. `git push origin main`
 6. Report the commit hash and the verification summary
 
+Spawned subagents (task-tool launches) never commit or push on their
+own: they finish their assigned edits and report, and the session that
+spawned them commits once, after combined verification, so history
+stays reviewable as one logical change.
+
 Exceptions (**do not** commit or push without asking):
 
 - The change is exploratory / speculative (you're not sure the user wants