docs: Add Zig package cache guidance

[lox]

Zig users can already route package source downloads through content-cache, but the README does not make that path obvious. That makes it easy to assume content-cache needs a new Zig-specific remote build cache protocol, even though Zig currently uses URL-based package fetches and local filesystem build caches.

This documents the supported setup: use the existing Git proxy for git+https dependencies, use /fetch for tarball dependencies, and keep ZIG_GLOBAL_CACHE_DIR on persistent CI storage for compile output reuse.

Validation:

• git diff --check
• mise run test

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 2844726b21

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Use a Zig-visible proxy URL for git dependencies

When the dependency URL in build.zig.zon is git+https://github.com/..., this git config url.*.insteadOf setting is ignored because Zig's package fetcher handles git+http(s) itself (it strips the git+ prefix and opens its own HTTP Git session in Package/Fetch.zig:initResource) rather than invoking the Git CLI. In that scenario zig build still contacts GitHub directly, so users following this setup get no /git caching and CI environments relying on the proxy/allowlist will still fail or bypass content-cache.

Useful? React with 👍 / 👎.