feat(eve): add disableSubagent helper

[muratsu]

Summary

• add disableSubagent() as a public eve helper
• omit disabled local or remote subagents from the compiled graph and model-visible capability surface
• document conditional subagent configuration and add a patch changeset

Testing

• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/public/definitions/subagent.test.ts src/compiler/normalize-manifest.test.ts
• pnpm --filter eve typecheck
• pnpm docs:check
• pnpm guard:invariants
• focused oxlint checks

[vercel]

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[ctgowrie]

Let’s make defineDynamic work with subagents instead. Then simply return no subagent when env is not set.