Implement the 015-add-jscpd-gate feature that adds copy-paste detection to the quality gate using jscpd with a 5% duplication threshold integrated into pnpm check

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

specs/015-add-jscpd-gate/quickstart.md

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+# Quickstart: Copy-Paste Detection Quality Gate
+
+## What This Feature Does
+
+Adds automated copy-paste detection to the project's quality gates. When a developer commits code, jscpd scans for duplicated code blocks and blocks the commit if duplication exceeds the configured threshold. Developers can also run the scan on-demand.
+
+## Key Commands
+
+```bash
+# Run copy-paste detection standalone
+pnpm jscpd
+
+# Run full quality gate (includes jscpd)
+pnpm check
+
+# Commit triggers the check automatically via Lefthook
+git commit -m "my changes"
+```
+
+## Configuration
+
+The jscpd configuration lives in `.jscpd.json` at the repo root. Key settings:
+
+- **threshold**: Max allowed duplication percentage (default: 5%)
+- **minLines**: Minimum lines for a duplicate block (default: 5)
+- **minTokens**: Minimum tokens for a duplicate block (default: 50)
+- **pattern**: File globs to scan (TypeScript/TSX)
+- **ignore**: Directories/files to skip (node_modules, dist, etc.)
+
+## How It Integrates
+
+```
+git commit → Lefthook pre-commit → pnpm check → knip + biome + tsc + vitest + jscpd
+```
+
+The `pnpm check` script is the single merge gate. jscpd is added to this chain, so it automatically runs as part of pre-commit hooks and any CI that uses `pnpm check`.