feat(workspace): add typed desktop client state

[decode2]

Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

Problem Description

The frontend lacks typed workspace command access and explicit identity/capability state, making safe UI integration impossible.

Proposed Solution

Add a typed Tauri workspace client, branded WorkspaceId/SessionId/UI TabId adapters, and deterministic capability/loading/error state with focused tests.

Affected Area

Other

Alternatives Considered

Directly wiring UI actions was rejected because it would blur three identity domains and risk duplicate terminal spawning.

Additional Context

Tracks terminal-ai-platform-expansion PR12a / task 4.2c1. Commit 7ae5b6b. Review lineage review-f2836bcdc014e846.