EKS regional delete removes a same-named cluster's TTL in other regions

> 🤖 Generated by the Agentic Engineer

## Evidence

A cluster's TTL is stored as one file per cluster name, `~/.ksail/clusters//ttl.json`, with no region in the path. `SaveClusterTTL`, `LoadClusterTTL` and `clusterTTLPath` all take only a name. `create`, `info` and `list` read and write it that way.

EKS cluster names are unique only within an AWS region. `DeleteEKSRegionState` (added in #6333) deliberately deletes this name-scoped TTL on every regional delete, so that a stale TTL cannot auto-delete or misconfigure a later same-named cluster. The same choice has a cost. Deleting the EKS cluster `demo` in `eu-north-1` also removes the TTL of a still-running `demo` in `us-east-1`.

This came up in review of devantler-tech/ksail#7028. That PR keeps same-named clusters' other state per region, but TTL was out of its scope.

## Affected audience and impact

EKS users who run same-named clusters in more than one region and rely on `--ttl`. After deleting one region's cluster, the other region's cluster silently loses its expiry. It is never auto-deleted, so billable resources can outlive the intended lifetime.

## Expected behaviour

A regional delete removes only that region's TTL. A same-named cluster in another region keeps its own expiry, and `info`/`list` show it.

## Acceptance criteria

- [ ] EKS TTL state is scoped by region, so deleting one region never removes another region's TTL.
- [ ] Existing name-scoped `ttl.json` files keep working, or are migrated safely, for non-EKS distributions and single-region EKS clusters.
- [ ] `create`, `info`, `list` and TTL-driven auto-delete read the region-scoped value for EKS.
- [ ] A test seeds TTLs for the same name in two regions, deletes one region, and asserts the other region's TTL survives.

## Rough size

Medium: a storage contract change plus consumer updates and compatibility for existing state.

Start by tracing SaveClusterTTL, LoadClusterTTL, clusterTTLPath, and DeleteEKSRegionState, then inspect how create, info, list, and TTL-driven auto-delete select state for EKS. Run the existing TTL and regional-delete tests before changing the storage contract. Done means same-named EKS clusters retain independent regional TTLs, while existing non-EKS and single-region state remains compatible.

Written by the indexing model from the issue text.