Builds the read/surface half of the per-session first-turn-tokens metric
(the capture — sessions table + turn_stats.session_id — landed separately).
A fresh claude session's first turn pays the full static prefix (system
prompt + tools + CLAUDE.md + first wake) as uncached input, so its
input_tokens is a clean proxy for prompt / CLAUDE.md sprawl — watching it
over time surfaces creep.
- stats.rs: add `Snapshot.first_turn_ctx: Option<u64>` populated by
`read_first_turn_ctx` — the agreed per-session derive (first turn,
`ORDER BY started_at LIMIT 1`, of the most recent session that started
in the window). Inert-until-capture: `.ok()` maps both "no fresh
session yet" and "older db without the sessions table" to None, the
same decoupling as read_bash_breakdown; the field is skipped from the
JSON when None. Pre-capture rows have a NULL session_id and are excluded.
- agent stats.js: add a "first-turn ctx" summary chip, guarded on a
numeric value so it stays hidden until capture has data.
clippy + cargo fmt clean; agent bundle builds.
8db8bd610f