feat(loops): Run a skill instead of instructions

[charlesvien]

Problem

A loop's prompt is a free-form instructions blob. Teams that already maintain skills have no way to point a loop at one, so the same playbook gets pasted into instructions and drifts from the skill.

Changes

The loop form's prompt step gains a source choice: write instructions or run a skill. Picking a local skill zips it (plus its skill dependencies) via workspace-server on save and uploads the set through the new PUT /loops/{id}/skill_bundles/ endpoint, and the loop's instructions become /skill-name plus an optional context field. The backend seeds those bundles into every fired run, where the sandbox agent-server already installs skill_bundle artifacts.

The loop detail page shows the attached skill with its snapshot digest and an "Update from local skill" button to re-snapshot after editing the skill. On hosts without a local filesystem (web) the picker is hidden; an attached skill can still be kept or detached.

Requires PostHog/posthog#73069 to be deployed first; until then the skill_bundles endpoint 404s and loop.skill_bundles is absent.

How did you test this?

Unit tests for the new instruction build/parse helpers and the form round-trip (skill mode validity, derived instructions, attached-snapshot mapping) via pnpm --filter @posthog/ui test. Full pnpm typecheck and Biome lint are clean. Not exercised against a live backend since the API only exists in the paired posthog PR.

Automatic notifications

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[trunk-io]

😎 This pull request was merged.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit c40e35a.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
packages/ui/src/features/loops/components/LoopDetailView.tsx:325
**Missing bundles crash loop views**

When the older backend described in this PR omits `skill_bundles`, this unconditional `.length` access throws during rendering. The edit path also indexes the absent field in `primaryLoopSkillBundle`, so users cannot view or edit loops until the backend deployment completes.

### Issue 2 of 2
packages/ui/src/features/loops/components/LoopForm.tsx:145-153
**Failed uploads leave incomplete loops**

When dependency resolution, local bundling, or bundle upload fails, the loop has already been persisted with `/skill-name` instructions, and this catch only shows a toast before navigating away. The loop can then fire without the matching skill artifact, causing the run to receive the invocation without the selected skill definition.

_{Reviews (1): Last reviewed commit: "let loops run a skill instead of instruc..." | Re-trigger Greptile}

[veria-ai]

PR overview

All previously flagged issues have been addressed. No open security concerns remain on this pull request.

Security review

No open security issues remain on this pull request.

Fixed/addressed: 4 · PR risk: 0/10

[posthog]

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