Scope workspace removal to the active workbench

[cinderzhan]

Summary

• Keep Host workspace registration intact when a workspace is removed inside a workbench. The removal now persists a hidden workspace ID for that workbench only; sessions in that workbench become ungrouped, while another workbench keeps the same workspace and its sessions grouped.
• Add a narrow UI workspace filter and delete handler seam to the Harness 0.2.0-rc.2 patch. Native conversation mode continues to use the existing global delete behavior. A failed scoped delete never falls back to global deletion.
• Exclude hidden workspaces from default/new-session choices, restore them on explicit re-add of the same folder, and migrate legacy workbench IDs and saved state safely.

Verification

• Clean npm ci replayed the equivalent 0.1.7-rc.2 patch in the local checkout.
• npx vitest run test/workbench-state.test.mjs test/workbench-client.test.mjs test/workbench-navigation.test.mjs (193 passed)
• npm test (1542 passed), npm run typecheck, npm run build, git diff --check
• The 0.2.0-rc.2 patch was applied to a pristine upstream package and the result was compared byte-for-byte with the intended files; node --check passed.

The local checkout uses Harness 0.1.7-rc.2; this PR targets main on 0.2.0-rc.2. Windows packaged UI validation remains for CI/QA.