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.

benches/batch.rs

@@ -1,7 +1,7 @@
 use criterion::{Criterion, criterion_group, criterion_main};
 use trueskill_tt::{
-    BETA, GAMMA, KeyTable, MU, P_DRAW, Rating, SIGMA, agent::Agent, batch::Batch,
-    drift::ConstantDrift, gaussian::Gaussian, storage::AgentStore,
+    BETA, Competitor, GAMMA, KeyTable, MU, P_DRAW, Rating, SIGMA, batch::Batch,
+    drift::ConstantDrift, gaussian::Gaussian, storage::CompetitorStore,
 };
 
 fn criterion_benchmark(criterion: &mut Criterion) {
@@ -11,13 +11,13 @@ fn criterion_benchmark(criterion: &mut Criterion) {
     let b = index_map.get_or_create("b");
     let c = index_map.get_or_create("c");
 
-    let mut agents: AgentStore<ConstantDrift> = AgentStore::new();
+    let mut agents: CompetitorStore<ConstantDrift> = CompetitorStore::new();
 
     for agent in [a, b, c] {
         agents.insert(
             agent,
-            Agent {
-                player: Rating::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
+            Competitor {
+                rating: Rating::new(Gaussian::from_ms(MU, SIGMA), BETA, ConstantDrift(GAMMA)),
                 ..Default::default()
             },
         );