fix(skills): stop skills updater throwing ENOENT on removed staging dir

[posthog]

Problem

The desktop app's skills auto-updater (PosthogPluginService) staged downloads into a shared <appData>/skills.new directory and then called readdir on it to validate the download — with no ENOENT guard and no per-run isolation. The updating flag only serializes runs within a single process, so a second workspace-server instance (another app window/project) — or the atomic swap rename — could delete skills.new out from under an in-flight run, and the readdir blew up with ENOENT ... scandir '.../skills.new'.

The failure is non-fatal (the updater retries on the next interval and bundled skills keep working), so the real cost was error-tracking noise from an exception that shouldn't be surfaced.

Changes

• Isolate staging per instance. The staging and backup dirs are now <skills>.new-<runId> / <skills>.old-<runId>, keyed on a stable per-process runId (process.pid). Concurrent instances no longer share a path, so they can't clobber each other's staging dir. The dirs stay on the same filesystem as the runtime skills dir, so the atomic-swap rename still works, and the stable pid lets the create-staging-dir step self-heal any leftover dir from an interrupted prior run.
• ENOENT-tolerant reads/swap (defense in depth). A missing staging dir is now treated as an empty, no-op cycle (skip the swap, retry next interval) rather than a thrown ENOENT — at both the validate-skills readdir and just before the swap rename.

How did you test this?

• Added a regression test that simulates a concurrent instance removing the staging dir mid-run and asserts the cycle is a silent no-op (no captureException, existing cache preserved, no false "updated" event) — it fails without the fix.
• Added a test asserting the shared .new path is never used and the per-instance staging/backup dirs are cleaned up on success.
• pnpm --filter @posthog/workspace-server test posthog-plugin → 40 passing.
• Biome lint clean on the changed files; typecheck clean for the changed files.

Automatic notifications

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

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Created with PostHog Code from an inbox report.

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 86d6003.}