fix-up(#4027): narrow coordinator guard to registry hard-reject set

Maintainer feedback (#4071 review): the original guard rejected
sisyphus and atlas as subagent targets even from team-mode where
resolveMember() intentionally calls resolveSubagentExecution with
allowPrimaryAgentDelegation: true. Per AGENT_ELIGIBILITY_REGISTRY
(src/features/team-mode/types.ts), only prometheus is hard-reject;
sisyphus and atlas are explicitly verdict: 'eligible' for team
membership.

Shrink COORDINATOR_AGENT_NAMES to ['prometheus'] so the guard
aligns with the registry's authoritative classification, document
the scoping rule in a comment, and add regression tests covering:

- sisyphus is NOT blocked by the coordinator guard (registry eligible)
- atlas is NOT blocked by the coordinator guard (registry eligible)
- prometheus IS blocked even when allowPrimaryAgentDelegation: true
  (registry hard-reject is authoritative)

Fixes the 5 zauc-mocks resolver tests that were locking in the
wrong rejection set (including 'allows delegating to a primary
agent when allowPrimaryAgentDelegation is enabled'). The one test
asserting the literal primary-agent error string for Prometheus
display-name was loosened to a regex that accepts either guard's
message, since prometheus is now caught by the coordinator path
which fires before the primary-agent lookup.

src/tools/delegate-task/constants.ts

@@ -352,9 +352,15 @@ export function isPlanFamily(category: string | undefined): boolean {
  * arbitrary subagent targets via task(). Delegating to these creates duplicate
  * orchestration and conflicting team state (issue #4027).
  *
+ * Scoped to AGENT_ELIGIBILITY_REGISTRY hard-reject entries only — sisyphus and atlas
+ * are explicitly marked `verdict: "eligible"` for team membership in the registry
+ * (src/features/team-mode/types.ts), so they are NOT included here. Adding them would
+ * conflict with the team-mode resolver's intentional `allowPrimaryAgentDelegation: true`
+ * opt-in.
+ *
  * Symmetric guard to the caller-eligibility check added by PR #4065 for team_create.
  */
-export const COORDINATOR_AGENT_NAMES = ["prometheus", "atlas", "sisyphus"]
+export const COORDINATOR_AGENT_NAMES = ["prometheus"]
 
 /**
  * Returns true when the given agent name refers to a coordinator/meta agent that