feat(storage, workspace, cli): Durable storage with workspace projection

[JeanMertz]

Every conversation now has a durable user-local copy stored under <user-data-dir>/workspace/<workspace-id>/. Non-local conversations are additionally projected into the workspace's .jp/conversations/ directory so they can be committed to version control alongside code. Local-only conversations (--local) are kept exclusively in user-local storage and never appear in the workspace directory.

The local indicator in jp conversation ls now shows three states: N for projected, Y (blue) for user-local only, and ext (magenta) for conversations present only in the workspace (committed by another contributor but not yet imported locally). Archive and unarchive operations now act on every storage root that holds the conversation, keeping both copies consistent.

On first startup for a workspace, JP migrates any legacy <name>-<id> user directories into the new workspace-ID layout (<id>) and imports existing workspace conversations into user-local storage. Conversations committed by other contributors after the one-time migration appear as ext and are imported lazily on first write. When both roots hold a conversation, metadata and stream are resolved independently by file mtime, with ties broken in favour of the durable user-local copy.

The Conversation::user field is removed; storage locality is now carried by Projection (the write intent on a lock) and derived at load time from StoragePresence. PersistBackend::write gains a Projection argument, and LoadBackend::load_conversation_ids is replaced by load_conversation_index which returns ConversationIndexEntry values pairing each ID with its presence. FsStorageBackend::with_user_storage no longer accepts a name parameter.

Closes: #733