Add AGENTS.md and CLAUDE.md for AI agent guidance

[alex-rawlings-yyc]

Documents architecture, commands, testing conventions, and mock internals so AI agents can work effectively in this codebase without re-deriving project-specific conventions.

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• AGENTS.md
• CLAUDE.md
• cspell.json

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[imnasnainaec]

@imnasnainaec reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: 2 of 3 files reviewed, all discussions resolved (waiting on alex-rawlings-yyc).

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AGENTS.md line 95 at r1 (raw file):

`@papi/backend` and `@papi/frontend` mocks are mutually exclusive: backend tests use `papi-backend.ts`, WebView tests use `papi-frontend.ts` + `papi-frontend-react.ts`. Each mock file ends with `export {}` so TypeScript treats it as a module.

### Mock internals

My first impulse is that the mock internals are too detailed here. But I'll try to give it a bit more thought tomorrow.