docs: check for existing Nextcloud capabilities before writing new logic

[moodyjmz]

Summary

Follow-up to #88. "Reuse before you write" only covered UI reuse — an @nextcloud/vue component, a design token, an existing src/ pattern. It said nothing about checking whether the server already exposes the behaviour, or whether a non-UI @nextcloud/* package already covers it.

• Broadens the preference ladder to four steps: an existing server feature (OCP interface or protocol-level capability) → the wider @nextcloud/* npm scope, not just @nextcloud/vue → @nextcloud/vue components/tokens → existing src/ patterns.
• The OCP check is concrete and grep-able: this app vendors the full OCP stub tree via composer require-dev (nextcloud/ocp → vendor/nextcloud/ocp/OCP/), so "check for an existing interface" is a real local grep, not an appeal to background knowledge an agent may not have.
• Adds a partial-fit escape valve wired into the existing "When the call is contestable, ask" rule, so the new instruction can't be used to justify contorting a bad fit into place.
• Folds the new rule's PR-description disclosure into the existing pre-PR checklist rather than adding another scattered "say X in the description" instance.

#100's DAV orderby/limit fix is the motivating example: the server already supported both via the DAV SEARCH grammar, so the fix was a one-file request change instead of custom client-side pagination. This rule is meant to make checking for that kind of existing capability the default first move, not a lucky diagnosis.

Process note

This draft went through an adversarial review pass before being opened. An earlier version hardcoded this app's @nextcloud/* dependency list (stale the moment a dependency changes — and a direct contradiction of the token guidance two paragraphs below it, which explicitly refuses to enumerate for that same reason), gave no verifiable method for the OCP check ("say you checked" isn't auditable), and used "capability" for two different things four lines apart. All three are fixed in this version.

Test plan

• N/A — docs only, no code paths affected