Add project scaffolding

Makefile, Dockerfile, CI workflow, prettier config, manifests for
Chrome (MV3) and Firefox (MV2), source directory structure, and
minimal test suite. All checks pass.

Makefile

@@ -0,0 +1,42 @@
+.PHONY: test lint fmt fmt-check check docker hooks build clean dev
+
+test:
+	@echo "Running tests..."
+	@timeout 30 yarn run test 2>&1
+
+lint:
+	@echo "Linting..."
+	@yarn run lint 2>&1
+
+fmt:
+	@echo "Formatting..."
+	@yarn run fmt 2>&1
+
+fmt-check:
+	@echo "Checking formatting..."
+	@yarn run fmt-check 2>&1
+
+check: test lint fmt-check
+
+build:
+	@echo "Building extension..."
+	@yarn run build 2>&1
+
+clean:
+	@rm -rf dist/
+
+dev:
+	@echo "Building in watch mode..."
+	@yarn run build --watch 2>&1
+
+docker:
+	@docker build -t autistmask .
+
+hooks:
+	@echo "Installing pre-commit hook..."
+	@mkdir -p .git/hooks
+	@echo '#!/usr/bin/env bash' > .git/hooks/pre-commit
+	@echo 'set -euo pipefail' >> .git/hooks/pre-commit
+	@echo 'make check' >> .git/hooks/pre-commit
+	@chmod +x .git/hooks/pre-commit
+	@echo "Pre-commit hook installed."