[FEATURE] ISM action to auto scale down writers on index migration for Reader/Writer Separation

[lawofcycles]

Is your feature request related to a problem?
In Reader/Writer Separation architecture, when indices migrate through ISM lifecycle states (e.g., Hot → Warm → Cold), writer shards (primary + write replicas) continue to consume resources even when no longer receiving write traffic.

For example, in a typical log management use case, Hot tier indices receive active writes for 7 days, then transition to Warm tier where writes become minimal or stop entirely. There's currently no automated way to optimize these resources while keeping indices searchable.

What solution would you like?
A new ISM action (e.g., optimize_for_search) that automatically scales down writer shards during state transitions.

• Leverage the existing _scale API (Search only replicas (scale to zero) with Reader/Writer Separation OpenSearch#17299) to set search_only: true
• Scale down writer shards (primary + write replicas) while maintaining search replicas
• Include prerequisite validation (Remote Store enabled, Segment Replication enabled, etc.)

What alternatives have you considered?

• Manual API calls: Users could manually call the _scale API for each index, but this doesn't scale well for clusters managing thousands of indices with ISM policies
• External automation: Building custom scripts/tools outside ISM, but this adds complexity and doesn't integrate with existing lifecycle management workflows

Do you have any additional context?

• I am interested in contributing this feature
• This addresses a follow-up item in the Reader/Writer Separation META issue: [META] Reader/Writer Separation OpenSearch#15306
• Related PR: Search only replicas (scale to zero) with Reader/Writer Separation OpenSearch#17299 (Search only replicas - scale to zero)