feat(eve): make integration setup headless-deterministic

[jseramn]

Summary

Related to #1786. Built-in integration setup always used an interactive Asker, so coding agents could not supply answers by key, and an unlinked Vercel project could open an interactive linking wizard mid-flow after other choices had already started mutating external state.

This slice adds headless + keyed answers to runIntegrationSetup, routes Discord through an explicit gather-then-perform split, and makes ensureVercelProject throw HumanActionRequiredError (vercel-link) in headless mode when no on-disk link exists instead of launching the interactive linker. Interactive CLI/TUI behavior is preserved. Linear/Slack/GitHub/Photon only pass headless through for the prerequisite; full gather→perform migration for those is follow-up.

Validation

• pnpm fmt
• pnpm lint
• pnpm --filter eve typecheck
• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/setup/integrations/runner.test.ts src/setup/integrations/discord/setup.test.ts src/setup/flows/ensure-vercel-project.test.ts src/setup/ask.test.ts (46 passed)
• Repro notes: research/deterministic-integration-setup-repro.md

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 if this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)

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