refactor(auth): centralize inference-credential routing in CredentialAuthority

One authority answers 'which credential may ride this request':
credential_class / manager_for / credential_for / bearer_resolver_for,
keyed by (platform, base_url). SessionCredential is an opaque type with
no production constructor, so a new call site cannot re-introduce the
session-bearer leak. Platform-scoped tests extended across all bearer
channels (session, aux, summary, subagent override).

Verified: cargo check --workspace --all-targets clean; kigi-shell and
kigi-tui suites green (6611+ tests).

crates/codegen/kigi-shell/src/agent/mvp_agent/tests.rs

@@ -1291,6 +1291,7 @@ mod subagent_spawn_context_tests;
 /// LEAK guard for the `api_key` channel (C1/C2), through the real
 /// `prepare_sampling_config_for_model` resolution path.
 mod api_key_channel_leak_tests;
+mod chokepoint_leak_tests;
 /// No load in flight and no session → the wait returns immediately
 /// (the caller then surfaces "unknown session id" exactly as before).
 #[tokio::test]