feat(a2a): publish shared schema package

[miyaontherelay]

Summary

• add a new published @relaycast/a2a package for shared A2A schemas and types
• switch the server A2A engine to import those schemas/types from the new package
• keep compatibility re-exports in packages/server/src/engine/a2a.ts so existing internal server imports keep working

Why

Sage and Cloud are currently vendoring a2a-schemas.ts from relaycast. That is the wrong long-term shape. Relaycast already owns the A2A schema truth, so it should publish a package both repos can depend on instead of copying schema files.

What this unlocks

• Sage can depend on @relaycast/a2a instead of vendoring schemas
• Cloud specialist-worker can depend on the same package
• Relaycast remains the canonical source of truth for A2A wire schemas

Validation

• npm run build --workspace @relaycast/a2a

Additional server workspace validation in this clean worktree was noisy due to broader existing workspace/environment typing issues (for example Worker globals / package resolution), so this PR keeps the scope bounded and preserves server compatibility via re-exports in engine/a2a.ts.

[github-actions]

Preview deployed!

Environment	URL
API	https://pr123-api.relaycast.dev
Health	https://pr123-api.relaycast.dev/health
Observer	https://pr123-observer.relaycast.dev/observer

This preview shares the staging database and will be cleaned up when the PR is merged or closed.

Run E2E tests

npm run e2e -- https://pr123-api.relaycast.dev --ci

Open observer dashboard

https://pr123-observer.relaycast.dev/observer