cli: show recent sessions (not commits) in `entire activity`

The entire.io Overview page renders a recent-sessions feed from GET /api/v1/me/sessions, but entire activity rendered a commits feed from /me/commits, so the two surfaces listed different things.

Make sessions the default recent-list in entire activity, matching the web: fetch /me/sessions (limit 50, mirroring the web's USER_OVERVIEW_RECENT_SESSIONS_LIMIT), group by local day of lastActivityAt (newest first), and render one row per session with the same fields the web row shows — display name, repo, public tag, agent badge, friendly model label, and checkpoint count. The stat cards, contribution heatmap, and repo chart (all from /me/activity) are unchanged.

Commits stay available behind entire activity --commits.

• Port entire.io's formatModel (frontend/src/lib/model.ts) to Go so model labels read identically (claude-opus-4-6 -> "Opus 4.6", gpt-4o -> "GPT-4o", gemini-2.0-flash -> "Gemini 2.0 Flash").
• Extract the shared always-fetch-activity-plus-a-list plumbing (fetchActivityWith) and day-grouping (groupItemsByDay) so the commit and session paths don't duplicate it.
• Routes through the existing runAuthenticatedActivityAPI, so /me/sessions goes to the caller's home entire-api cell with the data-API fallback.

Stacked on #1641.