Document composables as an established pattern in AGENTS.md, once #134 proves the model

Sequence

1. #134's PR 1 lands: the scroll-arrows extraction as proof of concept.
2. #134's PR 2 lands: deliberately extended as the model across the other identified clusters (create-from-template dialog, view-mode toggle, active-creator/routing sync).
3. Once that's done and the shape feels right in practice — not guessed at in advance — this issue's work begins.

What to write, once there's real code to point at

Most of the conventions should be a light pointer, not a prescriptive block — matching how AGENTS.md already handles this (e.g. "see TemplateSection.vue" for the onUnmounted cleanup pattern, rather than re-explaining the pattern in prose). Once 3-4 real composables exist in src/composables/, most of their shape is self-teaching by example: return individual refs (not a reactive() object), use the shared lifecycle test helper for composables with onMounted/onUnmounted.

One thing does need an explicit line rather than a pointer, because it's a process step invisible in the finished code: a reuse ladder specific to composables, extending AGENTS.md's existing "Reuse before you write" ladder (currently OCP → @nextcloud/* packages → @nextcloud/vue components/tokens → existing src/ patterns) with a new rung —

• Before writing a new composable: can an existing composable in src/composables/ be extended to cover it?
• If not, does @vueuse/core already have it? (Verified during #134's scoping that useScroll covers the scroll-arrows case exactly — this is a real, not hypothetical, first hit.)
• Only then write a new one.

That's a genuine trade-off analysis a reader can't infer just by reading the resulting code, so it earns its own line rather than relying on the code to teach it.

Related

• #134 — the code work this documents. Do not merge this issue's scope into that one.