chore: add knip for unused export/dependency detection

Adds `pnpm check:unused` script powered by knip. Initial run finds
17 unused files, 3 unused deps, 96 unused exports, and 117 unused
exported types — all candidates for cleanup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -14,6 +14,7 @@
     "test:e2e:email": "pnpm --filter @capakraken/web test:e2e:email",
     "test:scripts": "node --test scripts/*.test.mjs",
     "audit": "pnpm audit --audit-level=high",
+    "check:unused": "knip",
     "check:architecture": "node ./scripts/check-architecture-guardrails.mjs",
     "check:exports": "node ./scripts/check-workspace-exports.mjs",
     "check:imports": "node ./scripts/check-workspace-imports.mjs",
@@ -40,6 +41,7 @@
     "@capakraken/eslint-config": "workspace:*",
     "@capakraken/tsconfig": "workspace:*",
     "husky": "^9.1.7",
+    "knip": "^6.3.1",
     "lint-staged": "^16.4.0",
     "prettier": "^3.3.3",
     "turbo": "^2.3.3",