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.
