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/builtin-agents/athena-council-guard.ts

@@ -39,14 +39,15 @@ Each member requires \`model\` (\`"provider/model-id"\` format) and \`name\` (di
 After informing the user, **end your turn**. Do NOT try to work around this by using generic agents, the council-member agent, or any other fallback.`
 
 /**
- * Replaces Athena's prompt with a guard that tells the user to configure council members.
+ * Replaces Athena's orchestration prompt with a guard that tells the user to configure council members.
+ * The original prompt is discarded to avoid contradictory instructions.
  * Used when Athena is registered but no valid council config exists.
  */
 export function appendMissingCouncilPrompt(
   athenaConfig: AgentConfig,
   skippedMembers?: Array<{ name: string; reason: string }>,
 ): AgentConfig {
-  let prompt = (athenaConfig.prompt ?? "") + MISSING_COUNCIL_PROMPT
+  let prompt = MISSING_COUNCIL_PROMPT
 
   if (skippedMembers && skippedMembers.length > 0) {
     const skipDetails = skippedMembers.map((m) => `- **${m.name}**: ${m.reason}`).join("\n")

src/agents/builtin-agents/council-member-agents.ts

@@ -27,6 +27,7 @@ export function registerCouncilMemberAgents(
   const agents: Record<string, AgentConfig> = {}
   const registeredKeys: string[] = []
   const skippedMembers: SkippedMember[] = []
+  const registeredNamesLower = new Set<string>()
 
   for (const member of councilConfig.members) {
     const parsed = parseModelString(member.model)
@@ -40,16 +41,16 @@ export function registerCouncilMemberAgents(
     }
 
     const key = getCouncilMemberAgentKey(member)
+    const nameLower = member.name.toLowerCase()
 
-    if (agents[key]) {
+    if (registeredNamesLower.has(nameLower)) {
       skippedMembers.push({
         name: member.name,
-        reason: `Duplicate name: '${member.name}' already registered`,
+        reason: `Duplicate name: '${member.name}' already registered (case-insensitive match)`,
       })
       log("[council-member-agents] Skipping duplicate council member name", {
         name: member.name,
         model: member.model,
-        existingModel: agents[key].model ?? "unknown",
       })
       continue
     }
@@ -66,6 +67,7 @@ export function registerCouncilMemberAgents(
     }
 
     registeredKeys.push(key)
+    registeredNamesLower.add(nameLower)
 
     log("[council-member-agents] Registered council member agent", {
       key,