fix: hide loop builder instructions from chat

[MattPua]

Problem

Loop builder sessions show internal builder instructions as the user message in chat.

Changes

Before:

After:

• Show only the prompt entered by the user
• Pass builder guidance through hidden custom instructions
• Keep context metadata available to the agent

How did you test this?

• pnpm exec vitest run packages/ui/src/features/loops/loopBuilderPrompt.test.ts
• pnpm exec biome check packages/ui/src/features/loops/loopBuilderPrompt.ts packages/ui/src/features/loops/loopBuilderPrompt.test.ts packages/ui/src/features/loops/hooks/useLoopBuilderTask.ts
• pnpm --filter @posthog/ui typecheck
• Full repository typecheck attempted by pre-commit, but failed in apps/web on existing unresolved @posthog/agent and @posthog/host-trpc exports

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[trunk-io]

😎 Merged manually by @charlesvien - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit c6f0878.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
packages/ui/src/features/loops/hooks/useLoopBuilderTask.ts:38-39
**Unseeded Builder Cannot Start**

When the instructions are empty or whitespace-only, both `content` and `taskDescription` become empty strings. Task creation requires one of these fields to contain non-whitespace text, so it returns `Task description cannot be empty` before the hidden instruction to ask what the user wants can reach the agent; previously, the generated builder prompt kept this flow valid.

_{Reviews (1): Last reviewed commit: "fix: hide loop builder instructions from..." | Re-trigger Greptile}

[charlesvien]

/trunk merge

[posthog]

👋 Visual changes detected for this PR.

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