📝 Route every task through the full branching model

Drop the leaf-task exception in AGENTS.md: a task without subtasks was implemented on the current branch, bypassing create:branch / create:finish. Every task now opens and closes a branch, so the clean-tree / current-main / green-baseline preconditions and the post-merge verify always apply.

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- route every backlog task through the full branching model, dropping the leaf-task exception
 - move the `_` fallback to a second argument typed `Exclude<T, keyof handlers>`, so it receives only the unhandled keys, and infer handler returns from short, unannotated handlers
 
 ## [0.3.0] - 2026-09-18