test: cover UTF-8 file pipe regressions

[EfeDurmaz16]

Summary

Adds regression coverage for the UTF-8 file-read cases discussed in #242:

• jq file input redirected to a file preserves multibyte text
• custom commands can decode multibyte stdin piped from cat
• custom-command example docs now show decodeBytesToUtf8(ctx.stdin) for text operations

Verification

• pnpm --dir packages/just-bash exec vitest run src/commands/jq/jq.utf8-stdin.test.ts src/custom-commands.test.ts
• pnpm --filter just-bash typecheck
• pnpm lint
• pnpm --filter just-bash knip
• pnpm --filter just-bash build

Refs #242

[vercel]

@EfeDurmaz16 is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[CodyWatters]

Great find! just ran into this - thanks for opening this pr

[cramforce]

Looks great. Please sign your commit and run pnpm changeset!

[EfeDurmaz16]

Thanks! I’ll sign the commit and add a changeset now.

[EfeDurmaz16]

Done. Added the changeset and updated the PR to use a verified signed commit.

[EfeDurmaz16]

Done. I amended the commit with Signed-off-by and confirmed the changeset is present via pnpm exec changeset status --since=c08352c.

I also re-ran the focused regression coverage for this PR:

• pnpm --filter just-bash exec vitest run src/commands/jq/jq.utf8-stdin.test.ts src/custom-commands.test.ts
• pnpm exec biome check .changeset/dry-worms-mix.md examples/custom-command/README.md packages/just-bash/src/commands/jq/jq.utf8-stdin.test.ts packages/just-bash/src/custom-commands.test.ts

The remaining red checks are Vercel preview deployment authorization checks.

[cramforce]

Please re-push with a signed commit

[EfeDurmaz16]

Re-pushed with a GitHub-verified SSH-signed commit: cd4f1a5. No content changes beyond rewriting the commit signature. The remaining red checks are Vercel deployment authorization checks.