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/config/schema.test.ts

@@ -635,7 +635,7 @@ describe("Athena agent override", () => {
       agents: {
         athena: {
           council: {
-            members: [{ model: "openai/gpt-5.3-codex" }],
+            members: [{ model: "openai/gpt-5.3-codex", name: "GPT" }],
           },
         },
       },