Implement proactive release notifications with escalating cadence

[gaiksaya]

Is your feature request related to a problem? Please describe

The release manager must remember to check criteria repeatedly. Nothing nudges them when a deadline is approaching and criteria remain unmet. The team discovers gaps reactively — often too late to unblock without delaying the release.

Describe the solution you'd like

Create a release notifier Lambda triggered by EventBridge Scheduler:

1. Queries opensearch_release_schedule for active versions
2. Computes days_to_rc / days_to_release → determines cadence phase
3. Queries opensearch_release_state for current criteria
4. Applies R/Y/G rubric, posts to RM Slack channel with verdict + reasoning + "since last check" delta
5. Skips notification if nothing changed

Cadence (two-anchor model):

Phase	Window	Focus	Cadence
Pre-RC ramp-up	14–8 days before RC	Entrance criteria	Daily
Pre-RC final push	7–0 days before RC	Entrance criteria	Every 6h
RC → Release	Between RC and release	Exit criteria	Daily
Final 2 days	2–0 days before release	Exit criteria	Every 6h

Infra:

• EventBridge Scheduler rule (every 6h; Lambda decides whether to post)
• RM channel config in Secrets Manager

Describe alternatives you've considered

• Having the RM manually query OSCAR daily — rejected because the whole point is removing that burden.
• Fixed-rate notifications regardless of phase — rejected because early noise causes alert fatigue.

Acceptance Criteria

• Notifications fire at the correct cadence for each phase
• Message includes R/Y/G verdict, per-criterion breakdown, "since last check" delta, days remaining
• No notification sent when nothing changed since last post
• Cadence correctly anchors to rc_date for entrance criteria and release_date for exit criteria
• EventBridge rule visible in CloudFormation and Lambda invoked on schedule
• Configurable RM channel via Secrets Manager