feat: add dead code expansion gates

[bntvllnt]

Summary

• add opt-in check rules for dead files, unused types, unused members, and dependency hygiene
• add additive finding metadata: kind, confidence, evidence in JSON/SARIF
• sync schema, docs, LLM docs, and roadmap for CH-P1-05

Tests

• pnpm lint
• pnpm typecheck
• pnpm build
• pnpm test (32 files, 457 tests)
• pnpm verify:cli-real (pass=59 fail=0)
• git diff --check

Review

• /tmp/code-review-dead-code-expansion.json (CLEAN)