feat(installer): auto-configure athena council members based on available providers

The installer now detects which providers the user has (Anthropic, OpenAI,
Google, Copilot, OpenCode Zen) and generates council member config for Athena.
Requires at least 2 distinct providers; skips council config otherwise.
This implements the documented claim in configurations.md.

src/cli/council-members-generator.ts

@@ -0,0 +1,54 @@
+import type { ProviderAvailability } from "./model-fallback-types"
+
+export interface CouncilMember {
+  model: string
+  name: string
+}
+
+const COUNCIL_CANDIDATES: Array<{
+  provider: (avail: ProviderAvailability) => boolean
+  model: string
+  name: string
+}> = [
+  {
+    provider: (a) => a.native.claude,
+    model: "anthropic/claude-sonnet-4-5",
+    name: "Claude",
+  },
+  {
+    provider: (a) => a.native.openai,
+    model: "openai/gpt-5.2",
+    name: "GPT",
+  },
+  {
+    provider: (a) => a.native.gemini,
+    model: "google/gemini-3-flash",
+    name: "Gemini",
+  },
+  {
+    provider: (a) => a.copilot,
+    model: "github-copilot/gpt-5.2",
+    name: "Copilot GPT",
+  },
+  {
+    provider: (a) => a.opencodeZen,
+    model: "opencode/claude-sonnet-4-5",
+    name: "OpenCode Claude",
+  },
+]
+
+export function generateCouncilMembers(avail: ProviderAvailability): CouncilMember[] {
+  const members: CouncilMember[] = []
+
+  for (const candidate of COUNCIL_CANDIDATES) {
+    if (candidate.provider(avail)) {
+      members.push({ model: candidate.model, name: candidate.name })
+    }
+  }
+
+  if (members.length < 2) {
+    return []
+  }
+
+  return members
+}