Add testing docs

docs/local-testing.md

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+# Local Testing Guide
+
+This document explains how to run e2e tests locally using `act`.
+
+## Prerequisites
+
+1. **Docker** installed and running
+2. **act** installed ([install guide](https://github.com/nektos/act#installation))
+   ```bash
+   brew install act  # macOS
+   ```
+3. **1Password credentials** (see [Required Secrets](#required-secrets))
+4. Build action
+
+## Required env variables
+
+| Secret                     | Description           |
+| -------------------------- | --------------------- |
+| `OP_SERVICE_ACCOUNT_TOKEN` | Service Account token |
+| `VAULT`                    | Vault name or UUID    |
+
+## Building Before Testing
+
+If you've modified TypeScript code, rebuild before running E2E tests:
+
+```bash
+npm run build
+```
+
+## Testing
+
+### Run E2E tests using Service Account
+
+```bash
+act push -W .github/workflows/e2e-tests.yml \
+  -s OP_SERVICE_ACCOUNT_TOKEN="$OP_SERVICE_ACCOUNT_TOKEN" \
+  -s VAULT="$VAULT" \
+  -j test-service-account \
+  --matrix os:ubuntu-latest
+```
+
+## Run unit tests
+
+```bash
+npm test
+```