fix(ci): skip Slack notify steps when SLACK_WEBHOOK_URL is unset

[tommaso-castellani]

Problem

The three Notify Slack steps in .github/workflows/shadcn.yml curl the webhook unguarded:

curl -X POST --data "{...}" ${{ secrets.SLACK_WEBHOOK_URL }}

This repo has SLACK_WEBHOOK_URL set, so it stays green and the bug is invisible here. Every repo cloned from this template does not have it. There, the URL expands to nothing, curl exits 2 (curl: (2) no URL specified), and the run is marked failed — even when the shadcn update itself worked perfectly.

Four downstream repos have been failing this way every single day for over a week (template-test, template-clone, cardinal-core-sync-replay, dive-the-plan). On template-test the workflow did its whole job correctly, found an already-open PR, and then died posting to Slack.

Fix

• Lift the secret to a job-level env.SLACK_WEBHOOK_URL.
• Guard each notify step with env.SLACK_WEBHOOK_URL != '' so it self-skips when the secret is absent.
• Quote the URL in the curl call.

Behaviour in this repo is unchanged (the secret is set, so all three steps still fire). Repos without the secret now simply skip the notification instead of failing the run.

Same patch already applied to tommaso-castellani/dive-the-plan.

[nsosio]

do we still use this flow? can we drop it completely?

[tommaso-castellani]

Superseded — @nsosio asked to drop the shadcn workflow from the template entirely, so guarding the Slack steps is moot here. Closing in favour of a PR that removes .github/workflows/shadcn.yml.