ci: remove broken Copier Update workflow

[ichoosetoaccept]

Summary

Removes .github/workflows/copier-update.yml. It pins actions-ext/copier-update@v0, but that action has no v0 tag (no tags/releases at all — only main), so the workflow has never resolved the action. Every weekly run (cron Sundays 05:00 UTC) fails in ~3s at "Prepare all required actions" and emails a failure notice.

Why

The template-sync job has never actually run — it dies before the first step. Pure weekly noise with no upside. The root cause lives in the Copier template and is tracked separately.

Test plan

• No more Run failed: Copier Update emails from this repo.
• The workflow was self-contained; nothing else references it.

Refs DOT-598
Linear: https://linear.app/ismar/issue/DOT-598

Note

Remove broken Copier Update workflow that pins nonexistent actions-ext/copier-update@v0

Deletes copier-update.yml, which referenced actions-ext/copier-update@v0 — a tag that does not exist. The workflow was failing within seconds every week, generating noise emails across all repos generated from this template, and the template-sync feature has never functioned as a result.

🖇️ Linked Issues

Resolves DOT-598, which tracks the broken weekly Copier Update runs caused by the nonexistent @v0 tag.

^{Macroscope summarized fa53c25.}

[linear-code]

DOT-598

[macroscopeapp]

Approvability

Verdict: Approved

Removes a broken CI workflow that never functioned (pinned non-existent action tag). Pure CI cleanup with no runtime impact - only eliminates weekly failure email noise.

^{You can customize Macroscope's approvability policy. Learn more.}