fix(athena): address 5 audit findings — remove harmful thinking config, add error handling, improve tests

- Remove applyModelThinkingConfig and all callers (OpenCode handles thinking natively)

- Add try/catch error handling to prepare_council_prompt filesystem operations

- Add mode input validation to prepare_council_prompt tool

- Add 2 missing placeholder test cases (RETRY_FAILED_IF_OTHERS_FINISHED, CANCEL_RETRYING_ON_QUORUM)

- Remove redundant 'Do NOT use TodoWrite' prompt section from council member agent

src/agents/athena/index.ts

@@ -1,3 +1,2 @@
 export { createAthenaAgent, ATHENA_PROMPT_METADATA } from "./agent"
 export { createCouncilMemberAgent, COUNCIL_MEMBER_PROMPT, COUNCIL_SOLO_ADDENDUM, COUNCIL_DELEGATION_ADDENDUM } from "./council-member-agent"
-export { applyModelThinkingConfig } from "./model-thinking-config"