feat(palette): unify quick open into command palette with multi-source search

Merge file picker into command palette. Single Cmd+P entry searches
files, sessions, settings pages and commands; groups re-order by best
fuzzy score per source. Sessions show branch labels; git status is
lazily fetched for all session directories on open.

Drop QuickOpenDialog and Cmd+K shortcut.

packages/electron/main.mjs

@@ -2148,8 +2148,7 @@ const buildMacMenu = () => {
         },
         { type: 'separator' },
         { label: 'Settings', accelerator: 'Cmd+,', click: () => dispatchAction('settings') },
-        { label: 'Command Palette', accelerator: 'Cmd+K', click: () => dispatchAction('command-palette') },
-        { label: 'Quick Open…', accelerator: 'Cmd+P', click: () => dispatchAction('quick-open') },
+        { label: 'Command Palette', accelerator: 'Cmd+P', click: () => dispatchAction('command-palette') },
         { type: 'separator' },
         { role: 'services' },
         { type: 'separator' },