Stop reporting skill-update no-op cycles as errors

[joethreepwood]

Problem

PostHog Code's UpdateSkillsSaga runs on a periodic refresh (and around skill-store activity). Its two skill-download steps are intentionally non-fatal, but the validate-skills step then hard-threw "No skills found from any source" whenever the staging dir ended up empty. So a transient network blip surfaced a misleading, opaque exception via captureException — even though the user's existing skills cache and the bundled skills were fully intact and nothing was actually broken. This was the single most common skills-related error in error tracking.

Why: raised from a support thread after the error was hit while creating a skill in the skill store — it's noise that masks real failures and reads as scary to users.

Changes

• An empty download cycle is now treated as a no-op when a skills cache already exists: log a warning, keep the existing cache, skip the swap, and retry next interval — no exception reported.
• Only when there is genuinely no cached skills to fall back on do we throw, and now with a clear, actionable message (likely cause, that bundled skills still work, and that it retries automatically) instead of the opaque "No skills found from any source".
• skillsUpdated is only emitted when the cache actually changed.

How did you test this?

• pnpm exec vitest run on posthog-plugin.test.ts — 26/26 pass, including a new regression test (empty cycle with an existing cache stays silent and preserves skills) and an updated test asserting the genuine no-cache failure reports the clear message, not the old opaque one.
• Typecheck and Biome clean on the touched files.

Automatic notifications

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

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Created with PostHog from a Slack thread

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit d391bb4.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Stop reporting skill-update no-op cycles..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "Merge branch 'main' into posthog-code/fi..." | Re-trigger Greptile}