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/lib.rs

@@ -3,11 +3,11 @@ use std::{
     f64::consts::{FRAC_1_SQRT_2, FRAC_2_SQRT_PI, SQRT_2},
 };
 
-pub mod agent;
 #[cfg(feature = "approx")]
 mod approx;
 pub(crate) mod arena;
 pub mod batch;
+mod competitor;
 pub mod drift;
 mod error;
 pub(crate) mod factor;
@@ -20,6 +20,7 @@ mod rating;
 pub(crate) mod schedule;
 pub mod storage;
 
+pub use competitor::Competitor;
 pub use drift::{ConstantDrift, Drift};
 pub use error::InferenceError;
 pub use game::Game;