feat: surface closed sessions in the "+" tab menu

[adamatan]

Fixes #77

This PR solves a big usability issue I have: too many tasks and tabs open at the same time, just because I'm not sure I can close them. Adding the ability to reliably restore any closed tab (or archived task) helps me keep a focused environment, without the nagging fear that closing something means losing it for good.

UX

• Tab strip "+" (inside an open task): a new "Resume" section lists the last few secondary agent tabs you've closed in this task, each with its CLI icon and a relative timestamp ("2m ago"). Clicking one reopens it with the exact same session, so the agent picks the conversation back up instead of starting fresh. A "More…" row links to the full History view.
• Sidebar project "+" (the task launcher): the same "Resume" pattern, but for that project's recently archived tasks, restoring the whole task and switching to it, one click closer than going through History.
• Close confirmation, relaxed: closing a non-shell terminal/agent tab still asks for confirmation by default, but the dialog now has a "Don't ask again" checkbox. Once turned off (also available as a toggle in Settings → General), closes happen immediately and a toast tells you where the tab went, with an inline "Resume" button.

Technical change

• src/store/app.ts: new closedTabs: Record<taskId, ClosedTabEntry[]> slice, in-memory only (cleared on app restart, no new persistence/backend work). closeTab snapshots a closing secondary tab (title, cli, session id) into this list right before syncDurableTabs would otherwise forget it for good; the main tab and shell tabs are excluded since they either auto-resume already or have no session. A new resumeClosedTab action removes the entry and reopens it by constructing a fresh TerminalTab with the original sessionId pre-set, letting the existing resume-by-id logic in TerminalPane do the rest (no changes needed there).
• src/components/task/TabBar.tsx: the "Resume" dropdown section, reusing the existing DropdownItem/DropdownLabel primitives and the same icon-resolution helpers as the rest of the menu.
• src/components/sidebar/ProjectActionsMenuItems.tsx: same pattern, sourced from s.tasks filtered to archived && project_id === projectId, sorted most-recent-first (mirrors HistoryView's sort), restoring via the existing task_restore IPC + setActiveTask.
• src/lib/closeTab.ts / src/store/prefs.ts: new confirmBeforeCloseAgentTab preference (default on), gating whether the confirmation dialog blocks the close; a toast with a bound "Resume" action fires on the fast path.
• CHANGELOG.md: added an [Unreleased] entry (not a version bump, that stays the maintainer's call at release time).
• Fixed a real bug caught during manual testing: a useApp(s => s.closedTabs[task.id] ?? []) selector was minting a new array reference on every render, which (Zustand v5 uses useSyncExternalStore internally) caused React's "Maximum update depth exceeded" crash. Fixed with a stable shared empty-array constant.
• Fixed a second bug from code review: the "Don't ask again" checkbox was being persisted even when the close was cancelled (Escape/Cancel/click-outside), silently disabling future confirmations against the user's actual intent. Now only persists on actual confirmation.

Test coverage

• npx tsc -b and npm test (242 tests, all passing) after every change, including after rebasing onto the latest workspace→task rename migration.
• Two rounds of automated code review (code-reviewer subagent), each finding and fixing one real bug (see above).
• Live end-to-end verification via the dev automation bridge against a real running app: spawned a secondary agent tab, closed it, confirmed it appeared in both the toast and the "+" > Resume dropdown, clicked Resume through real DOM events, and confirmed via the debug log that the PTY respawned with --resume <the original session id> (not a fresh session), with a screenshot showing the terminal itself printing that resume argument. Repeated the same for the sidebar's archived-task Resume (restored an archived task, confirmed it un-archived and became the active task, Claude Code launched with "Welcome back").
• No automated test coverage added for the new UI paths themselves (dropdown rendering, toast wiring). The existing resume.integration.test.ts covers the underlying resume-by-id mechanism this reuses, but the new Resume-menu UI and the closedTabs capture/eviction logic are only covered by the manual e2e pass above. Happy to add unit coverage for closedTabs capture/cap/resume in app.test.ts if useful before merge.