ci(templates): LHCI workflow is dead on arrival in monorepo layout

[spencer-osbrjp]

Problem

Scaffold copies audit templates into webDir (pkg/scaffold.go — the cfg.Audit != "none" block). In monorepo layout webDir is apps/web, so the Lighthouse workflow lands at apps/web/.github/workflows/lhci.yml. GitHub only runs workflows from the repo root's .github/workflows/, so scaffolded monorepo projects silently get no Lighthouse CI at all.

The workflow's assumptions are also flat-layout-shaped even if it were moved:

• run steps (install, build, lhci autorun) execute at the repo root with no working-directory: apps/web.
• The affected-pages step (ci(templates): LHCI workflow should audit only the pages a PR affects #113) diffs paths like src/pages/**, but in a monorepo changed files are prefixed apps/web/src/pages/**, so every diff falls through to the full-audit fallback.
• An apps/api-only PR can't change frontend Lighthouse scores but would still trigger a full audit — the inverse of the resource optimization we just shipped.

Proposal

When cfg.Layout.IsMonorepo():

• Emit the workflow at the repo root .github/workflows/ (keep lighthouserc.json in apps/web).
• Template a path prefix into the workflow: working-directory: apps/web on run steps (or cd apps/web), apps/web/src/pages (Astro) / apps/web/app/pages (Nuxt) as pages_dir, and strip the prefix when mapping routes.
• Add paths: ['apps/web/**', 'packages/**', ...] (or extend paths-ignore) so backend-only PRs skip the audit entirely.

Context

Follow-up to #112 and #113 — pre-existing placement bug noticed while implementing the affected-pages narrowing (PR #114).