feat(workspace): add lifecycle switcher

[decode2]

Pre-flight Checks

• I searched existing issues and found no duplicate
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Problem Description

Workspace commands and typed client state exist, but users cannot invoke lifecycle actions from the desktop UI.

Proposed Solution

Add a minimal workspace switcher supporting create, select, close, restart, and recover while preserving separate workspace, session, and UI tab identities.

Affected Area

Other

Alternatives Considered

Session adoption is intentionally deferred to PR12c to avoid duplicate PTY spawning.

Additional Context

Tracks PR12b / task 4.2c2. Commit f514911. Review lineage review-7888978dea736d4e.