refactor(api): rename Agent to Competitor and .player field to .rating

Competitor holds dynamic per-history state (message, last_time) for
someone competing; its configuration lives in a Rating.

AgentStore renamed to CompetitorStore to match. The internal
`clean()` free function's parameter name changed from `agents` to
`competitors` for consistency.

Local variable names (agent_idx, this_agent) inside history.rs are
left unchanged — they represent abstract identifiers, not Competitor
instances.

Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

src/competitor.rs

@@ -0,0 +1,50 @@
+use crate::{
+    N_INF,
+    drift::{ConstantDrift, Drift},
+    gaussian::Gaussian,
+    rating::Rating,
+};
+
+/// Per-history, temporal state for someone competing.
+///
+/// Renamed from `Agent` in T2; the former `.player` field is now
+/// `.rating` to match the `Player → Rating` rename.
+#[derive(Debug)]
+pub struct Competitor<D: Drift = ConstantDrift> {
+    pub rating: Rating<D>,
+    pub message: Gaussian,
+    pub last_time: i64,
+}
+
+impl<D: Drift> Competitor<D> {
+    pub(crate) fn receive(&self, elapsed: i64) -> Gaussian {
+        if self.message != N_INF {
+            self.message
+                .forget(self.rating.drift.variance_delta(elapsed))
+        } else {
+            self.rating.prior
+        }
+    }
+}
+
+impl Default for Competitor<ConstantDrift> {
+    fn default() -> Self {
+        Self {
+            rating: Rating::default(),
+            message: N_INF,
+            last_time: i64::MIN,
+        }
+    }
+}
+
+pub(crate) fn clean<'a, D: Drift + 'a, C: Iterator<Item = &'a mut Competitor<D>>>(
+    competitors: C,
+    last_time: bool,
+) {
+    for c in competitors {
+        c.message = N_INF;
+        if last_time {
+            c.last_time = i64::MIN;
+        }
+    }
+}