i18n: translate the interview modes (plan/practice/debrief) to Japanese

**Time:** ~30–45 min · **Difficulty:** Beginner · **Native Japanese speaker** career-ops ships the three interview-prep modes (plan / practice / debrief) in English, French, and Spanish — but not yet in Japanese. This adds the Japanese versions so Japanese-speaking job-seekers get interview prep in their own language. ### What to do 1. Create `modes/ja/interview/` and translate the three files from the English source: `modes/interview/plan.md`, `modes/interview/practice.md`, `modes/interview/debrief.md`. 2. Keep every technical token **unchanged** — file paths, `{slug}` templates, the STAR+R structure, code spans, enums. Translate only the human-facing prose. 3. Keep the `##` section headers in **English** — this is the convention set by the fr/es interview modes and it helps the agent parse the structure consistently. 4. Register the new directory in SYSTEM_PATHS: one line in `update-system.mjs`, mirroring how `modes/es/interview/` is registered. ### Acceptance - `modes/ja/interview/{plan,practice,debrief}.md` exist, in faithful native Japanese. - Technical tokens byte-identical to the English source. - `node validate-system-paths-coverage.mjs` passes (the new dir is registered). ### Pattern to mirror Copy the shape of **`modes/es/interview/`** (or `modes/fr/interview/`) exactly — same files, same anchors, same frontmatter. PRs #1526 (French) and #1523 (Spanish) are your reference implementations. *Reserved for a first-time contributor — if you're new here and speak Japanese, this one's yours. Comment to claim it and I'll help you land it.*