feat(athena): harden council config — mandatory name, guard prompt, no-crash duplicates

- Add council config guard prompt: when Athena has no valid council members,
  inject a STOP instruction telling the user how to configure council members
  instead of failing messily with generic agents
- Make council member 'name' field mandatory (was optional with auto-naming)
- Remove humanizeModelId and UPPERCASE_TOKENS — no more fragile auto-naming
- Replace throw on duplicate names with log + skip (graceful degradation)
- Update schema, types, tests (87 pass), and documentation

src/agents/athena/types.ts

@@ -1,7 +1,7 @@
 export interface CouncilMemberConfig {
   model: string
   variant?: string
-  name?: string
+  name: string
   temperature?: number
 }