[FEATURE] Add Zig syntax highlighting for code blocks (ready-made grammar: highlightjs-zig)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Note: #15175 requested this previously but was auto-closed by the stale bot as not-planned with no maintainer response, and is now locked. Its closing message invited filing a new issue — this is that issue.

Problem Statement

Fenced ```zig code blocks render as plain monochrome text in the terminal — no syntax highlighting at all, while mainstream languages highlight fine. Anyone using Claude Code on Zig codebases reads every explanation, snippet, and diff discussion unstyled, which is noticeably harder to scan than other languages.

Proposed Solution

Bundle a Zig grammar in the highlight.js setup Claude Code uses for code-block rendering (the changelog mentions highlight.js 11). Zig is not in highlight.js core, so it never arrives via library upgrades — but a ready-made third-party grammar exists: https://github.com/fwx5618177/highlightjs-zig. Registering that grammar should be a small, self-contained addition. Precedent: Cedar policy file highlighting (.cedar) was added as a one-off grammar by request.

Alternative Solutions

Currently tagging Zig snippets as ``rust to borrow approximate keyword coloring (fn, const, pub`). Works, but it's a hack and mis-colors Zig-specific syntax.

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI).