feat: opt-in ETag read cache for github storage

[openrijal]

Stacked on #3 — please merge #3 first. Until #3 lands, the diff here shows both #3's commits and the new cache commit (`6c3103f`). After #3 merges, only the cache commit will remain in the diff.

Summary

Adds an in-process LRU read cache (cap 64) keyed by `owner/repo@ref:path`, storing parsed JSON alongside the GitHub response `ETag`. When enabled, reads send `If-None-Match` on subsequent requests and short-circuit on 304. GitHub returns 304 without spending rate-limit budget, so this is a meaningful latency + quota win on repeated reads of the same resource — admin list re-renders, navigation between detail and list views, etc.

Disabled by default at every layer.

Enabling

Globally (recommended for trusted deployments where you own all writes):

```ts
// nuxt.config.ts
runtimeConfig: {
autoadmin: {
github: { cacheReads: true },
},
}
```

Per-resource (overrides the global default):

```ts
register({
kind: 'array',
key: 'blogs',
storage: {
kind: 'github',
owner: 'me',
repo: 'cms',
path: 'data/blogs.json',
cacheReads: true,
},
// ...
})
```

Why opt-in, not always-on

• Module-scoped state is undesirable in multi-tenant shared isolates.
• A stale cache could hide a manual repo edit (e.g. someone edits the JSON in GitHub's web UI between admin reads).
• Some deployments deliberately want every read to hit GitHub for audit reasons.

Writes always invalidate the cached entry unconditionally (success or 409 conflict), so the cache invariants stay correct regardless of the flag. The gate only controls whether reads populate and consult the cache.

What it does NOT do

• No TTL — entries live until they're evicted by LRU or invalidated by a write. ETag-conditional requests are cheap (304 is free), so a TTL would be a regression. If you want time-based eviction, call `clearGithubReadCache()` from your own scheduler.
• No cross-isolate sharing. Each worker/isolate has its own cache. For Cloudflare Pages this means warm isolates benefit, cold starts don't.
• Does not cache writes, error responses, or 404s.

API additions (all optional)

• `GithubReadOptions.cacheReads?: boolean`
• `GithubJsonRepositoryOptions.cacheReads?: boolean`
• `JsonStorageConfig` (github variant): `cacheReads?: boolean`
• `runtimeConfig.autoadmin.github.cacheReads?: boolean`
• `clearGithubReadCache(): void` exported from `server/utils/githubContents.ts` for tests

Test plan

• Standalone typecheck clean (only Nitro auto-imports flagged).
• Manual: enable, hit the same endpoint twice, confirm second response is 304 from GitHub.
• Manual: write a row, then read — confirm cache is bypassed and new content is returned.
• CI / lint on this repo.