docs(desktop): add layout regression remediation issue block

[Whiteks1]

Summary

• add a new Desktop/UI issue block for the remaining layout/workbench regression
• track empty-pane collapse, active-surface focus containment, and runs-family density separately
• record the continuity in .agents/session-log.md

Issues

• Closes desktop(ui): collapse empty panes and restore primary workbench ownership #342
• Closes desktop(ui): enforce stronger active-surface focus and context containment #343
• Closes desktop(ui): rebalance runs-family density and right-rail space budget #344

Validation

• planning-only slice
• diff limited to .agents/

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[Whiteks1]

desktop-smoke is failing before the harness can read
esult.json (ENOENT under /tmp/quantlab-desktop-smoke-*). This reproduced on the initial run and on a rerun, and is unrelated to the .agents/-only diff in this PR. Tracking the blocker in #346 before merging.