refactor(athena): remove dead code from phases 2, 3, 5 pipeline

Remove 9 files (913 lines) from the code-driven synthesis pipeline that
was superseded by the agent-driven approach in phases 6-8.

Phases 3/5 built: collectCouncilResults → formatForSynthesis →
buildSynthesisPrompt → formatFindingsForUser → buildDelegationPrompt.

Phases 6-8 replaced with: launch → background_output → Athena
synthesizes in conversation → switch_agent. The old pipeline was
never wired into runtime and all consumers were other dead code.

Also simplifies executeCouncil to return CouncilLaunchResult (task IDs
+ failures) instead of reading stale task status via collectCouncilResults.

Deleted: council-result-collector, synthesis-types, synthesis-prompt,
synthesis-formatter, findings-presenter, delegation-prompts (+ 4 tests).
Cleaned: CouncilMemberStatus, AgreementLevel, CouncilMemberResponse,
CouncilExecutionResult types from types.ts.

src/agents/athena/index.ts

@@ -3,10 +3,4 @@ export * from "./agent"
 export * from "./model-parser"
 export * from "./council-prompt"
 export * from "./council-orchestrator"
-export * from "./council-result-collector"
-export * from "./synthesis-types"
-export * from "./synthesis-prompt"
-export * from "./synthesis-formatter"
-export * from "./findings-presenter"
-export * from "./delegation-prompts"
 export * from "../../config/schema/athena"