Add PR writing guidance for AI assistants to AGENTS.md

[rcantin-w]

What does this change?

Ports the PR-writing guidance from wellcomecollection.org's AGENTS.md (wellcomecollection.org#13493) into this repo, adapted for content-api. Adds a "Writing style" and "AI-assisted PRs" subsection to the Pull Requests section: casual/terse tone, ticket-ref placement, no "no alarms needed" conclusions in the risk section, and a collapsed <details> block for genuinely non-obvious reasoning aimed at future AI-assisted debugging.

Also fixes the existing PR section to use the template's actual heading ("Have we considered potential risks?" instead of "Risks").

The writing-style guidance references British spelling via a .github/copilot-instructions.md file, which this repo didn't have. So this PR also adds a trimmed-down version of wellcomecollection.org's copilot-instructions.md - just the parts that apply to a backend API: British spelling, duplication/code reuse, Prismic API-ID naming conventions (relevant since content-api reads from the same Prismic repo), and PR review guidance for stale TODOs, dead code, and doc updates. Dropped the Next.js-specific bits (accessibility, component naming, workspace structure) since they don't apply here.

How to test

Docs-only change - read through the updated Pull Requests section and copilot-instructions.md and check the guidance reads clearly and matches how we actually want PRs written here.

Have we considered potential risks?

None - it's documentation that only affects how future PR descriptions and Copilot reviews get written, not any runtime behaviour.

For AI / future debugging

The "how to test" example originally pointed at a hosted domain (api-stage, then api-dev) by mistake - content-api's local dev server actually runs on port 3002 (api/package.json's dev script sets PORT=3002 explicitly, overriding the ?? 3000 fallback in server.ts), so the example links localhost:3002 instead.

Whether GitHub's AI-attribution footer is suppressed for this repo (the reason wc.org gives for its own _Written by Claude Code._ signature) couldn't be confirmed via the GitHub API - no such toggle showed up in repo or org settings. Kept the signature line anyway since it's cheap insurance and matches the sibling repo's convention.

Written by Claude Code.

[Copilot]

🟡 Changes recommended

The new risk-writing guidance conflicts with the existing template's request for mitigation and alarm details.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Adds guidance for PR writing and AI-assisted reviews, plus backend-specific coding standards for the Content API.

Changes:

• Expands PR description style and AI-assisted authoring guidance.
• Adds British English, reuse, Prismic naming, and review guidance.
• Corrects the PR risk-section heading.

File summaries

File	Description
AGENTS.md	Adds PR writing and AI-assisted PR guidance.
.github/copilot-instructions.md	Adds Content API coding and review standards.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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