refactor(builder): abstract session metadata storage (#2258)

## Description

Closes #2068.

### Background

The builder session registry was directly backed by a local
`sessions.json` file. This coupled session metadata management to local
storage and prevented multiple builder replicas from sharing the same
session metadata.

### Changes

- Convert `SessionStore` from a local file implementation into a storage
interface.
- Move the existing JSON persistence behavior into `LocalSessionStore`.
- Add a fail-fast `RemoteSessionStore` placeholder for future shared
storage implementations.
- Allow custom `SessionStore` implementations to be injected through
`BuilderBootstrap.Builder`.
- Keep `LocalSessionStore` as the default for single-node deployments.
- Require a shared `SessionStore` when the builder uses a distributed
`AgentStateStore`.
- Fail during startup instead of silently falling back to local storage
in distributed mode.