feat(understand): auto-detect conversation language on first run

[BozhengLong]

Summary

On the first /understand run in a project (no --language flag, no stored outputLanguage), the skill now detects the conversation language and — only when it is non-English — confirms once before generating, persisting the choice to config.json. This fixes the silent English-default surprise for non-English users, who previously only discovered the mismatch after paying the full cost of an analysis run.

The confirmation gate fires only when the detected language is non-English and no language has been chosen yet. English conversations follow the exact same silent en path as before. The --language flag and stored config both take priority over detection. Non-interactive invocations fall back to the detected language with a notice instead of blocking.

Linked issue(s)

Refs #141 (the --language foundation this builds on). Complementary to #374 (dashboard runtime switcher) — this sets the generation-time default rather than the view-time UI.

How I tested this

• Traced all 5 scenarios against the edited step 3.6: (1) Chinese conversation → detect + confirm → zh persisted; (2) re-run uses stored config, no gate; (3) English conversation → silent, no gate (no regression); (4) --language ja → flag wins; (5) override at gate (zh detected, type en) → en.
• pnpm test — not run; this is a markdown-only change (SKILL.md prompt + README), no code paths touched.
• Manual reasoning smoke test.

Versioning

• Version bumped in all five manifests (2.7.5 → 2.7.6) — user-visible behavior change.

Design + plan included under docs/superpowers/ (consistent with the repo's existing specs there).

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