fix: add charset=utf-8 to .txt responses via _headers

[Debbl]

Summary

https://hono.dev/llms.txt (and llms-full.txt / llms-small.txt) is served in production with a bare Content-Type: text/plain, while wrangler dev returns text/plain; charset=utf-8 locally.

$ curl -sI https://hono.dev/llms.txt | grep -i content-type
content-type: text/plain

$ curl -sI http://localhost:8787/llms.txt | grep -i content-type
Content-Type: text/plain; charset=utf-8

The difference comes from wrangler: its local asset server appends charset=utf-8 to every text/* type, but the production Static Assets service only maps the extension to text/plain. Since these files contain non-ASCII content (🔥, CJK), clients that fall back to ISO-8859-1 show mojibake.

Fix

Add public/_headers with a rule for *.txt:

/*.txt
  Content-Type: text/plain; charset=utf-8

public/ is copied verbatim into .vitepress/dist, so the file ships with the normal build and is picked up by Workers Static Assets.

Verification

Ran wrangler dev against the built dist with this file included:

[wrangler:info] ✨ Parsed 1 valid header rule.
$ curl -sI http://localhost:8799/llms.txt | grep -i content-type
Content-Type: text/plain; charset=utf-8

Note: after deploy, the Cloudflare edge cache (cf-cache-status: HIT) may need to expire or be purged before the new header shows up.

🤖 Generated with Claude Code

[yusukebe]

LGTM!

[yusukebe]

@Debbl

This fix is meaningful. Thanks!