feat(#2608): show session count in agent stats window

Adds a 'sessions' chip to the per-agent stats summary panel showing how
many fresh claude sessions started within the selected time window.

Backend (hive-agent/src/stats.rs):
- New optional field `session_count: Option<u64>` on `Snapshot`
  (skip_serializing_if = None — inert-until-data, same pattern as
  first_turn_ctx). Counts rows in the `sessions` table whose
  started_at falls within the window; returns None when the table
  doesn't exist on an older db.
- New `read_session_count(conn, from)` helper (rusqlite::Result so the
  caller maps Err to None).
- Extracted per-row accumulation loop into `TurnAccum` struct +
  `push()` method to keep `snapshot()` under the too_many_lines limit.

Frontend (frontend/packages/agent/src/stats.js):
- New 'sessions' chip added to renderSummary, guarded by
  `typeof s.session_count === 'number'`, placed before the
  existing first-turn-ctx chip.

frontend/packages/agent/src/stats.js

@@ -123,6 +123,12 @@ window.Chart = Chart;
         ['reminders pending',   fmtInt(s.reminder_stats.pending)],
       );
     }
+    // Session count: fresh claude sessions started in the window (each
+    // new-session or auto-compaction-fallback mints one). Omitted until
+    // the sessions table exists (older db).
+    if (typeof s.session_count === 'number') {
+      chips.push(['sessions', fmtInt(s.session_count)]);
+    }
     // First-turn ctx: input tokens of the most recent fresh session's
     // first turn — the cold system-prompt + CLAUDE.md cost, a sprawl
     // proxy. Omitted from the JSON (and so absent here) until the