feat(app): show recent sessions in the empty command palette

[opencode-agent]

Summary

• Show up to five recently updated root sessions in a Recent sessions group when the command palette opens with an empty query, in both the in-session and home palettes.
• Typed session searches keep their 100 ms debounce and Session group label. The empty-query list loads right away and skips archived sessions.
• Commands stay first, so the default selection and Enter behavior don't change. Sessions that are already open keep their existing open-tab marker.

Follow-up to #51973, addressing feedback that the palette only shows commands such as "New session", "New terminal", and "Toggle review" instead of recent sessions.

Testing

• bun run typecheck in packages/app
• bun script/lint-changed.ts (0 problems)
• Unit tests for src/shell and src/home (126 passing)
• Added a command-palette.spec.ts keeper case that opens the empty palette and checks that the recent session is listed, isn't selected by default, and opens on click. CI runs the e2e suite.

Requested by: @Brendonovich (Brendan via Slack)