shell: add 'Close all other tabs' to the tab context menu

[miljanm]

Feature

The tab context menu gains Close all other tabs: it keeps only the right-clicked tab in its pane and makes it the pane's active tab.

Semantics:

• Pane-scoped. Only siblings in the clicked tab's pane close; tabs in other panes are untouched, matching the menu's existing pane-scoped actions (Split / Move / Close pane).
• Never a no-op item. The item renders only when a sibling tab exists, in both the single-pane strip and the per-pane strips.
• One reversible gesture. A single undo snapshot ("Closed other tabs" toast) restores every closed sibling at once — the same pattern as CLOSE_PANE.
• No auto-return branch. The kept tab survives, so the pane can never empty.

Implementation

• paneModel.js: a pure closeOtherTabs(ws, tabKey) (same-reference no-op when the tab is unknown or already alone) plus a CLOSE_OTHER_TABS reducer case with one undo slot.
• Shell.jsx: one gated menu item between "Close tab" and "Close pane".
• Tests: reducer behavior (keep / activate / undo / no-op) and pane-scoping in paneModel.test.js; the menu-item contract in workspaceUi.test.js.

Verification

Unit: the Shell test suite passes (151 tests). Live: drove the running shell end-to-end — opened three tabs, right-clicked one, picked "Close all other tabs"; only the clicked tab remained, in both the menu screenshot and the resulting strip. Undo semantics are covered by the reducer tests.

[hamzamerzic]

Reviewed the complete diff. The action is pane-scoped, same-reference on no-ops, restores the entire prior pane state through one undo snapshot, and leaves other panes untouched. The tests cover the owning model and the menu gate.

[miljanm]

Thanks for the thorough review and the approval! Nothing further needed from our side — ready to merge whenever it suits you.

[hamzamerzic]

Reviewed and merged to main through #304, preserving your authorship. The integrated version keeps the Close all other tabs action and its pane-level coverage. Thanks, Miljan.