configurable worker pool and release callback

[xytan0056]

Configurable workspace pool and release() cleanup

Why?

orchestrator calls ws.Release(), but workspace doesn't know about repo_manager stuff. So either we make orchestrator call repomanager to release by passing WS info, which is too much hassle. A better approach to add callback to WS, let it invoke repo manager's release logic.

also make worker pool size configurable with default to 1

What?

• workspace_pool_size added to RepositoryConfig — validated > 0 at startup, defaults to 1 if unset programmatically.
• workspace now takes an OnRelease callback to return itself to the pool, eliminating an unnecessary type.

Test Plan

CI unit test
local manual tests

Issue