i18n: refine KO/EN strings and drop unused keys

What & Why

Refines the KO/EN dictionaries and removes dead keys.

• KO policy — GitHub status badges stay English as fixed API-enum labels(Added/Removed/Modified/Renamed, Merged/Open/Closed, Private/Public), while navigation, actions, and descriptions are Korean.
• EN polish — capitalize file-status badges to match the other badges, sentence-case the expand/collapse actions, and drop the desktop-only "from the left" wording in selectFile.
• Cleanup — remove unused keys: code.branch, pulls.filesChanged / pulls.additions / pulls.deletions.
• Docs — AGENTS.md now states that API-enum status badges stay English in both locales, so they aren't "fixed" as untranslated strings later.

Related Issue

N/A

How to Verify

1. Browse in KO (/ko/...) — repo list, file viewer, commits, PR list & detail tabs: status badges show in English, everything else in Korean.
2. Browse in EN — file-status badges read Added/Removed/Modified/Renamed; expand/collapse buttons are sentence case.
3. npm run typecheck && npm run build pass (removing unused keys doesn't break the Dictionary type).

Checklist

• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, i18n:)
• Translations added to both locales/ko.json and locales/en.json (if UI text changed)
• Tested on mobile viewport (if UI changed)