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)

assets/oh-my-opencode.schema.json

@@ -5252,7 +5252,8 @@
                       },
                       "name": {
                         "type": "string",
-                        "minLength": 1
+                        "minLength": 1,
+                        "pattern": "^[a-zA-Z0-9][a-zA-Z0-9 .\\-]*$"
                       },
                       "temperature": {
                         "type": "number",