fix(athena): address 9 council-audit findings — dead code, bugs, and hardening

Fixes from multi-model council audit (7 members, 19 findings, 9 selected):

- Use parseModelString() for cross-provider Anthropic thinking config (#3)
- Update stale AGENTS.md athena directory listing (#4)
- Replace prompt in appendMissingCouncilPrompt instead of appending (#5)
- Extract duplicated session cleanup logic in agent-switch hook (#6)
- Surface skipped council members when >=2 valid members exist (#9)
- Expand fallback handoff regex with negation guards (#11)
- Remove dead council-member agent from agentSources and tests (#12)
- Make runtime council member duplicate check case-insensitive (#14)
- Fix false-positive schema tests by adding required name field (#18)

src/agents/AGENTS.md

@@ -51,11 +51,10 @@ agents/
 ├── momus.ts               # Plan review
 ├── atlas/agent.ts         # Todo orchestrator
 ├── athena/                # Multi-model council orchestrator
-│   ├── agent.ts           # Athena agent factory
+│   ├── agent.ts           # Athena agent factory + system prompt
 │   ├── council-member-agent.ts  # Council member agent factory
-│   ├── model-parser.ts    # Model string parser
-│   ├── types.ts           # Council types
-│   └── index.ts           # Barrel exports
+│   ├── model-thinking-config.ts  # Per-provider thinking/reasoning config
+│   └── model-thinking-config.test.ts  # Tests for thinking config
 ├── types.ts               # AgentFactory, AgentMode
 ├── agent-builder.ts       # buildAgent() composition
 ├── utils.ts               # Agent utilities