refactor(storage, workspace, cli): Implement layered storage backend

[JeanMertz]

Replace Workspace's Option<Storage> with four non-optional trait objects — PersistBackend, LoadBackend, LockBackend, and SessionBackend — eliminating pervasive Option branching throughout the workspace layer.

The root problem was that at least 15 methods on Workspace branched on self.storage.as_ref(), yielding three different failure modes for the same underlying condition: some silently no-oped, others returned errors, and others returned None. A disable_persistence boolean added a third dimension of optionality, and ConversationMut carried an Option<writer> that callers could not inspect.

The new jp_storage::backend module introduces:

• FsStorageBackend — wraps Storage, implements all four traits, and retains filesystem-specific path accessors (storage_path, user_storage_path) outside the trait surface.
• InMemoryStorageBackend — fully in-memory implementation for tests and future non-filesystem environments.
• NullPersistBackend — silently discards all writes; used for --no-persist and error-path persistence suppression.
• NoopLockGuard — a no-op ConversationLockGuard for contexts where real locking is not needed.

Workspace now holds persist, loader, locker, and sessions as always-present trait objects. Workspace::new initialises them with NullPersistBackend + InMemoryStorageBackend; callers opt into filesystem storage by calling with_backend(fs). The old builder chain Workspace::new(root).persisted_at(&storage)?.with_local_storage()? is replaced by constructing FsStorageBackend first, then passing it in. disable_persistence() retains its signature but now swaps the persist field for NullPersistBackend instead of flipping a boolean.

ConversationLock and ConversationMut lose their internal optionality: writer and lock_guard are always present. The if let Some(writer) branches in flush and Drop are gone — NullPersistBackend handles the no-write case through the trait.

jp_workspace::persist is deleted; the traits are now owned by jp_storage. Workspace::sanitize delegates to
LoadBackend::sanitize, which returns an empty report for in-memory backends. remove_ephemeral_conversations is reimplemented on Workspace using LoadBackend::load_expired_conversation_ids and PersistBackend::remove, preserving the fast-path metadata reader.

In jp_cli, Ctx gains an fs_backend: Option<Arc<FsStorageBackend>> field. Path queries and config loading that previously went through ctx.workspace.storage_path() now go through ctx.storage_path() or ctx.fs_backend. cleanup_stale_files and
build_partial_from_cfg_args receive the typed backend reference explicitly, keeping filesystem concerns out of the Workspace API.

The RFD 073 design document is added alongside the implementation.