fix: remove cacheDir override and node_modules CI caching

[mvvmm]

Summary

Removes the cacheDir: ".astro-cache" override from astro.config.ts, reverting to Astro's default node_modules/.astro. This was originally added in #31192 to work around a stale-cache problem caused by CI caching the entire node_modules directory — the dependency-only cache key (pnpm-lock.yaml + package.json hashes) didn't account for content/source changes, so stale Astro content-layer state persisted across runs.

Instead of working around the problem by moving the cache directory, this PR removes the root cause: the direct node_modules caching in all GitHub Actions workflows. All jobs now rely solely on setup-node's cache: pnpm (which caches the pnpm store, not node_modules itself) and run pnpm install --frozen-lockfile unconditionally. The targeted Astro assets cache is kept but updated to use the default path (node_modules/.astro/assets).

This also aligns the repo with Workers Builds' built-in Astro caching, which hardcodes node_modules/.astro as the cache directory.

Files changed:

• astro.config.ts — removed cacheDir override and .astro-cache from Vite watch ignored paths
• .gitignore — removed .astro-cache/ entry
• .github/workflows/ci.yml — removed node_modules cache/restore steps from all 5 jobs; updated Astro assets cache path to node_modules/.astro/assets
• .github/workflows/publish-production.yml — same
• .github/workflows/bonk.yml, bigbonk.yml, anchor-link-audit.yml — removed node_modules cache/restore steps

Screenshots (optional)

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