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Pull the per-repo unit of work out of the interval loop so a future webhook
receiver can trigger a refresh for a single repo by its GitHub coordinates,
not just the ticker iterating over managed entries.

• Registry.FindManaged(owner, name) resolves a managed mirror case-insensitively;
  it deliberately never returns the unmanaged -cwd working repo, which must not
  be mirror-fetched (mirror-fetch detaches HEAD).
• Syncer.syncEntry now returns an error instead of logging-and-swallowing;
  syncOnce logs failures, and the new SyncRepo(owner, name) wrapper resolves the
  entry and delegates, returning ErrRepoNotManaged for un-onboarded repos.

Pure refactor: the interval loop behaves identically. Sets up the webhook-driven
refresh stack.

Stack

💡 Notice: This PR is part of a stack merge into branch stack-merge-stack-1782266218.

• PR refactor(web): remove sample alice/bob stacks from repo provider #1313
  • PR feat(server): drive deploy posture from STACKIT_ENV, not $PORT #1314
    • PR refactor(api): extract Syncer.SyncRepo for per-repo sync triggers #1315 👈
      • PR feat(api): add GitHub webhook signature + push parsing helper #1316
        • PR feat(api): add GitHub webhook receiver for evented refresh #1317
          • PR feat(api): coalesce webhook-driven syncs per repo #1318
            • PR feat(api): add manual sync endpoint for on-demand refresh #1319
              • PR feat(server): default sync interval to 5m and document evented refresh #1320
                • PR feat(server): index repos by owner/repo for GitHub-style routes #1322
                  • PR feat(server): serve repos at /repos/{owner}/{repo} path routes #1323
                  • PR feat(web): GitHub-style path URLs for repos, branches, stacks, PRs #1324

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Merged via consolidation into #1326 by jonnii