WorkspaceBuilder: define a public builder protocol and entry points

[tony]

Summary

Define the public contract for custom workspace builders after the classic dotted-path selection work lands.

Related: #1054, #1058.

Proposed behavior

• Define a public builder protocol or interface that documents constructor inputs, .build(), .session, plugin lifecycle compatibility, append/detached behavior, progress callbacks, and script-output callbacks.
• Keep sync builders supported first while making the interface shape compatible with future async builders.
• Add package entry point discovery, for example a tmuxp.workspace_builders group, so packaged builders can be distributed without asking users to reference implementation module paths.
• Keep dotted-path configuration from the base PR as the simple and transparent path for most users.
• Document how tmuxp resolves builder names, dotted paths, and entry points without changing the classic default.

Acceptance criteria

• The builder contract is represented in public code and docs.
• Existing WorkspaceBuilder satisfies the contract.
• Tests cover a minimal custom builder, an entry-point-discovered builder, and invalid builder failures.
• The interface does not require async execution in this stage.