Add CI and quality gates

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly

.github/workflows/quality.yml

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+name: Quality
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  duplication:
+    name: Duplication (jscpd)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+      - run: >
+          npx --yes jscpd .
+          --threshold 3
+          --reporters consoleFull
+          --ignore "**/target/**,**/_build/**,**/node_modules/**,**/dist/**,**/*.lock,src/parser.c,src/grammar.json,src/node-types.json"
+
+  file-size:
+    name: File size
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: bash scripts/check-file-size.sh
+
+  typos:
+    name: Typos
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: crate-ci/typos@master

.github/workflows/security.yml

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+name: Security
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  secrets:
+    name: Secret scan (TruffleHog)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: trufflesecurity/trufflehog@main
+        with:
+          extra_args: --only-verified

scripts/check-file-size.sh

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+#!/usr/bin/env bash
+# Fails when a tracked source file exceeds the size ceiling.
+# Guideline: target ~300 lines per file; hard ceiling 800 before splitting.
+set -euo pipefail
+LIMIT="${FILE_SIZE_LIMIT:-800}"
+fail=0
+while IFS= read -r f; do
+  [ -f "$f" ] || continue
+  lines=$(wc -l < "$f")
+  if [ "$lines" -gt "$LIMIT" ]; then
+    echo "::error file=$f::$f has $lines lines (limit $LIMIT)"
+    fail=1
+  fi
+done < <(git ls-files \
+  '*.ml' '*.mli' '*.rs' '*.ts' '*.js' '*.go' '*.py' '*.java' \
+  | grep -vE '(^|/)(target|_build|node_modules|dist)/' || true)
+[ "$fail" -eq 0 ] && echo "All source files within the $LIMIT-line ceiling."
+exit "$fail"