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/athena/model-thinking-config.test.ts

@@ -52,4 +52,30 @@ describe("applyModelThinkingConfig", () => {
       expect(result).toBe(BASE_CONFIG)
     })
   })
+
+  describe("given a Claude model through a non-Anthropic provider", () => {
+    it("returns thinking config for github-copilot/claude-opus-4-6", () => {
+      const result = applyModelThinkingConfig(BASE_CONFIG, "github-copilot/claude-opus-4-6")
+      expect(result).toEqual({
+        ...BASE_CONFIG,
+        thinking: { type: "enabled", budgetTokens: 32000 },
+      })
+    })
+
+    it("returns thinking config for opencode/claude-opus-4-6", () => {
+      const result = applyModelThinkingConfig(BASE_CONFIG, "opencode/claude-opus-4-6")
+      expect(result).toEqual({
+        ...BASE_CONFIG,
+        thinking: { type: "enabled", budgetTokens: 32000 },
+      })
+    })
+
+    it("returns thinking config for opencode/claude-sonnet-4-6", () => {
+      const result = applyModelThinkingConfig(BASE_CONFIG, "opencode/claude-sonnet-4-6")
+      expect(result).toEqual({
+        ...BASE_CONFIG,
+        thinking: { type: "enabled", budgetTokens: 32000 },
+      })
+    })
+  })
 })

src/agents/athena/model-thinking-config.ts

@@ -1,4 +1,5 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
+import { parseModelString } from "../../tools/delegate-task/model-string-parser"
 import { isGptModel } from "../types"
 
 export function applyModelThinkingConfig(base: AgentConfig, model: string): AgentConfig {
@@ -6,10 +7,12 @@ export function applyModelThinkingConfig(base: AgentConfig, model: string): Agen
     return { ...base, reasoningEffort: "medium" }
   }
 
-  const slashIndex = model.indexOf("/")
-  const provider = slashIndex > 0 ? model.substring(0, slashIndex).toLowerCase() : ""
+  const parsed = parseModelString(model)
+  if (!parsed) {
+    return base
+  }
 
-  if (provider === "anthropic") {
+  if (parsed.providerID.toLowerCase() === "anthropic" || parsed.modelID.startsWith("claude")) {
     return { ...base, thinking: { type: "enabled", budgetTokens: 32000 } }
   }