feat(ci): add GitHub Actions pipeline with quality checks

Add continuous integration workflow that runs on all PRs and pushes
to main. The pipeline validates code quality through five sequential
checks: install dependencies, lint, typecheck, test, and build.

Author: prosdev

.github/CI.md

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+# Continuous Integration
+
+## Overview
+This project uses GitHub Actions for continuous integration. The CI workflow runs automatically on all pull requests and pushes to the `main` branch.
+
+## CI Workflow
+The CI workflow (`.github/workflows/ci.yml`) runs the following checks in sequence:
+
+1. **Install dependencies** - `yarn install --frozen-lockfile`
+2. **Type check** - `yarn typecheck` - Validates TypeScript types across all packages
+3. **Test** - `yarn test` - Runs all unit tests (currently 39 tests)
+4. **Build** - `yarn build` - Verifies all packages build successfully
+
+All checks must pass before code can be merged to `main`.
+
+> **Note:** Linting will be added in a future update when migrating to Biome.
+
+## Environment
+- **Node version:** 22.20.0
+- **Yarn version:** 1.22.18
+- **OS:** Ubuntu (latest)
+
+## Running CI Checks Locally
+Before pushing your changes, you can run the same checks locally:
+
+```bash
+# Run all checks
+yarn install --frozen-lockfile
+yarn typecheck
+yarn test
+yarn build
+
+# Or run them individually as needed
+```
+
+## Troubleshooting
+
+### yarn.lock out of sync
+If CI fails with a frozen lockfile error:
+1. Make sure you've committed your `yarn.lock` file
+2. Run `yarn install` locally and commit the updated `yarn.lock`
+3. Never manually edit `yarn.lock`
+
+### Test failures
+Run `yarn test` locally to debug failing tests. For watch mode during development, use `yarn tdd`.
+
+### Type check failures
+Run `yarn typecheck` locally to see TypeScript errors.
+
+### Build failures
+Run `yarn build` locally to reproduce build issues.
+
+## Branch Protection
+The `main` branch is protected and requires:
+- All CI checks to pass
+- Pull request reviews (if configured)
+- Branches to be up to date before merging
+
+## Adding New Checks
+To add additional CI checks, edit `.github/workflows/ci.yml` and add new steps under the `steps` section.
+

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  ci:
+    name: CI
+    runs-on: ubuntu-latest
+    env:
+      NX_CLOUD_DISTRIBUTED_EXECUTION: false
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '22.20.0'
+          cache: 'yarn'
+
+      - name: Install dependencies
+        run: yarn install --frozen-lockfile
+
+      - name: Setup Nx cache
+        uses: actions/cache@v3
+        with:
+          path: .nx/cache
+          key: ${{ runner.os }}-nx-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-nx-
+
+      - name: Type check
+        run: yarn typecheck
+
+      - name: Test
+        run: yarn test
+
+      - name: Build
+        run: yarn build
+