Add README

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

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+# 3CX Ring Group Scheduler
+
+AWS Lambda that automatically updates the after-hours and holiday forwarding destinations on a 3CX ring group daily at 8am ET.
+
+## How it works
+
+- Two EventBridge rules fire daily (one for EST, one for EDT) to handle DST
+- Lambda checks if it's actually 8am Eastern — skips if not
+- Reads the weekly schedule from `config/schedule.yaml`
+- Authenticates to 3CX V20 XAPI via OAuth2
+- Updates the ring group's "office closed" and "holiday" routing
+
+## Schedule config
+
+Edit `config/schedule.yaml` to change the weekly schedule or add exception dates:
+
+```yaml
+ring_group_number: "800"
+
+weekly_schedule:
+  Monday: "114"
+  Tuesday: "115"
+  Wednesday: "114"
+  Thursday: "115"
+  Friday: "116"
+  Saturday: "100"
+  Sunday: "100"
+
+exceptions:
+  "2026-12-25": "100"
+```
+
+## Setup
+
+### 1. Create 3CX API client
+
+In 3CX Admin Console > Integrations > API, create a new client with XAPI access enabled.
+
+### 2. Store credentials in SSM
+
+```bash
+aws ssm put-parameter --name /3cx-scheduler/domain --value "yourcompany.3cx.us" --type SecureString
+aws ssm put-parameter --name /3cx-scheduler/client_id --value "your-client-id" --type SecureString
+aws ssm put-parameter --name /3cx-scheduler/client_secret --value "your-secret" --type SecureString
+```
+
+### 3. Deploy
+
+```bash
+sam build
+sam deploy --stack-name ring-scheduler-3cx --resolve-s3 --capabilities CAPABILITY_IAM --parameter-overrides Timezone=America/New_York
+```
+
+## Manual invocation
+
+```bash
+# Run using today's schedule
+aws lambda invoke --function-name 3cx-ring-group-scheduler \
+  --payload '{"force": true}' \
+  --cli-binary-format raw-in-base64-out output.json
+
+# Override to a specific extension
+aws lambda invoke --function-name 3cx-ring-group-scheduler \
+  --payload '{"force": true, "override_extension": "100"}' \
+  --cli-binary-format raw-in-base64-out output.json
+```