docs: upgrade openspec workflow (#85)

AGENTS.md

@@ -1,24 +1,3 @@
-<!-- OPENSPEC:START -->
-
-# OpenSpec Instructions
-
-These instructions are for AI assistants working in this project.
-
-Always open `@/openspec/AGENTS.md` when the request:
-
-- Mentions planning or proposals (words like proposal, spec, change, plan)
-- Introduces new capabilities, breaking changes, architecture shifts, or big performance/security work
-- Sounds ambiguous and you need the authoritative spec before coding
-
-Use `@/openspec/AGENTS.md` to learn:
-
-- How to create and apply change proposals
-- Spec format and conventions
-- Project structure and guidelines
-
-Keep this managed block so 'openspec update' can refresh the instructions.
-
-<!-- OPENSPEC:END -->
 
 # User Guide