refactor: move collection slice into workspace

[hhhjin]

Summary

• move collection state and actions into the workspace slice
• update workspace tree, lifecycle, and fs flows to use workspace-owned collection state
• relocate collection tests under workspace and remove standalone collection exports

Validation

• pnpm --filter @mdit/store test
• pnpm --filter @mdit/store ts:check
• pnpm ts:check:desktop
• pnpm test:packages

Review

• review gate found no actionable issues in the reviewed packages/store scope

[gemini-code-assist]

Code Review

This pull request refactors the store by integrating the CollectionSlice directly into the WorkspaceSlice. This involves moving collection-related state and actions into the workspace domain, updating the store configuration in index.ts and core.ts, and adjusting various action creators and tests to reflect the new unified structure. Regarding the feedback, the use of a broad type cast in the setCollectionState helper was noted as a potential area for more specific typing, although it is currently constrained by CollectionStoreState.