feat(TASK-061): assert COYOTE_ROLE on all session/turn events; fix two stale comments

Cover the remaining role-carrying events with test_sink assertions:
session.started (ctx-held role), session.resumed and session.ended
(session-held role), turn.interrupted and turn.failed. Correct the
mcp_factory race comment (the loser fires a second plain connected and
overwrites the winner's map entry, not a reconnect) and the role_extras
doc (derived roles fall through to the session-held role).

src/hooks.rs

@@ -714,7 +714,8 @@ pub(crate) fn base_envs_parts(
 /// only two families that carry the variable. Resolved at fire time, because
 /// roles change mid-session (`.role`, `.exit role`, temp roles): the role
 /// held directly on the context wins, then the name of a role a session has
-/// absorbed. Derived roles have no name and report nothing.
+/// absorbed. Derived roles have no name; resolution then falls through to
+/// the session-held role, if any.
 pub fn role_extras(ctx: &RequestContext) -> Vec<(&'static str, String)> {
     ctx.role
         .as_ref()