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/storage/mod.rs

@@ -1,5 +1,5 @@
-mod agent_store;
+mod competitor_store;
 mod skill_store;
 
-pub use agent_store::AgentStore;
+pub use competitor_store::CompetitorStore;
 pub(crate) use skill_store::SkillStore;