Add duplicate code detection to pre-commit hook

Uses jscpd to detect code clones in shell scripts.
Blocks commit if duplication exceeds 5% threshold.
Suggests extracting common code to scripts/lib/common.sh.

.githooks/pre-commit

@@ -51,4 +51,38 @@ if [[ ${#staged_shell_files[@]} -gt 0 ]]; then
   fi
 fi
 
-printf 'shell_check passed. Proceeding with commit.\n'
+printf 'shell_check passed.\n'
+
+# Run duplicate code detection
+printf 'Running duplicate code detection...\n'
+
+JSCPD_BIN="${HOME}/.local/node_modules/.bin/jscpd"
+if [[ -x "$JSCPD_BIN" ]]; then
+  # Run jscpd and capture output
+  # Using threshold to fail if duplication exceeds limit
+  # --min-lines 10: minimum 10 lines to consider a clone
+  # --min-tokens 50: minimum 50 tokens to consider a clone
+  # --threshold 5: fail if more than 5% duplication
+  jscpd_output=$("$JSCPD_BIN" \
+    --pattern "**/*.sh" \
+    --min-lines 10 \
+    --min-tokens 50 \
+    --threshold 5 \
+    --reporters "console" \
+    --ignore "**/node_modules/**,**/.git/**" \
+    . 2>&1) || jscpd_exit=$?
+
+  if [[ ${jscpd_exit:-0} -ne 0 ]]; then
+    printf '\n%s\n' "$jscpd_output"
+    printf '\nCommit aborted: code duplication exceeds threshold (5%%).\n' >&2
+    printf 'Consider extracting common code to scripts/lib/common.sh\n' >&2
+    printf 'To see all duplicates: %s --pattern "**/*.sh" --min-lines 5 .\n' "$JSCPD_BIN" >&2
+    exit 1
+  fi
+  printf '  ✓ Duplication check passed (below 5%% threshold)\n'
+else
+  printf '  ⚠ jscpd not installed, skipping duplication check\n'
+  printf '    Install with: npm install --prefix ~/.local jscpd\n'
+fi
+
+printf 'All checks passed. Proceeding with commit.\n'