fix(athena): council review fixes — delegation bug, dead code, test coverage

- Add background_output to council-member allowlist (fixes delegation deadlock)
- Replace empty catch with error logging in prepare-council-prompt
- Remove unnecessary type assertion in agent.ts
- Remove dead hasAgentToolRestrictions function
- Fix incorrect test assertions (undefined vs false semantics)
- Add barrel export for athena module
- Add guard function test coverage (5 tests)
- Add parity test for triple-sync restrictions (9 tests)

src/agents/athena/council-member-agent.ts

@@ -52,6 +52,7 @@ export function createCouncilMemberAgent(model: string): AgentConfig {
     "lsp_diagnostics",
     "ast_grep_search",
     "call_omo_agent",
+    "background_output",
   ])
 
   // Explicitly deny TodoWrite/TodoRead even though `*: deny` should catch them.