Consolidate to one canonical storage layout, defer presets

[bordumb]

Problem

StorageLayoutConfig in layout.rs supports multiple presets (default, Radicle, Gitoxide) with different ref prefixes:

• Default: refs/auths/keys, refs/auths/identity
• Radicle: refs/rad/auths/keys, refs/rad/auths/identity
• Gitoxide: refs/gitoxide/auths/keys, refs/gitoxide/auths/identity

The widget currently only knows about the default layout. Supporting multiple layouts in the widget means either:

• Trying all 3 prefixes (expensive API calls)
• Requiring users to specify which layout their repo uses
• Making the widget layout-aware via configuration

None of these are great pre-launch.

Proposal

1. Ship with one canonical layout (the current default: refs/auths/registry)
2. Remove or gate the Radicle/Gitoxide presets behind a feature flag
3. Add preset support later when there's actual demand from those ecosystems

Why this matters

Every additional layout multiplies the test matrix and widget complexity. Pre-launch, we should optimize for one working path rather than three partially-tested ones.