feat(eve): support structured output for GitHub turns

[marcoshernanz]

Description

Adds turn-scoped structured output to GitHub channel deliveries.

• Lets GitHubInboundResult return outputSchema for webhook issue, pull-request, and comment turns.
• Lets cross-channel receive(channel, ...) pass the same schema for scheduled or proactive retries.
• Accepts Standard Schema or raw JSON Schema and normalizes it into durable JSON data.
• Scopes schemas to the current conversation turn while preserving runtime continuations and task-mode HITL contracts.
• Documents consuming validated results from the existing result.completed agent hook with trusted session auth.

Sessions remain durable and in conversation mode. This does not add a channel-specific result callback.

Closes #214.

Relationship to #207

#207 modifies cross-channel receive for empty deliveries. This PR touches the same receive path; whichever lands second should rebase and resolve that small overlap without changing either behavior.

How did you test your changes?

• pnpm lint
• pnpm fmt
• pnpm typecheck
• pnpm build
• pnpm test — 3,895 unit tests and 369 integration tests passed
• pnpm test:scenario — 260 passed, 15 platform-specific skips
• pnpm docs:check
• pnpm guard:invariants
• pnpm guard:fixtures
• pnpm check:deps
• pnpm changeset status
• NODE_ENV=test pnpm --filter agent-channels test:e2e — 2 evals and 5 gates passed

Credentialed workspace-wide E2E, model-driven TUI, and Vercel deployment checks could not complete because the required AI Gateway and Vercel credentials are unavailable locally. The relevant channel E2E passed in deterministic mock mode.

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant
• I added a changeset because this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)

[vercel]

@marcoshernanz is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[marcoshernanz]

Superseded by #221, which uses an upstream branch so the credential-gated CI jobs can run in trusted repository context.