feat: upgrade remaining MiniMax M2.5 fallbacks to M2.7-highspeed

Upgrade the secondary MiniMax fallback entries in librarian and explore
agents from M2.5 to M2.7-highspeed, completing the M2.5→M2.7 migration.

- librarian: minimax-m2.5 → minimax-m2.7-highspeed (2nd fallback)
- explore: minimax-m2.7 → minimax-m2.7-highspeed (2nd), minimax-m2.5 → minimax-m2.7 (3rd)
- Update corresponding test assertions

src/shared/model-requirements.test.ts

@@ -80,7 +80,7 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
 
     const second = librarian.fallbackChain[1]
     expect(second.providers[0]).toBe("opencode")
-    expect(second.model).toBe("minimax-m2.5")
+    expect(second.model).toBe("minimax-m2.7-highspeed")
 
     const tertiary = librarian.fallbackChain[2]
     expect(tertiary.providers).toContain("anthropic")
@@ -106,11 +106,11 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
 
     const secondary = explore.fallbackChain[1]
     expect(secondary.providers).toContain("opencode-go")
-    expect(secondary.model).toBe("minimax-m2.7")
+    expect(secondary.model).toBe("minimax-m2.7-highspeed")
 
     const tertiary = explore.fallbackChain[2]
     expect(tertiary.providers).toContain("opencode")
-    expect(tertiary.model).toBe("minimax-m2.5")
+    expect(tertiary.model).toBe("minimax-m2.7")
 
     const quaternary = explore.fallbackChain[3]
     expect(quaternary.providers).toContain("anthropic")

src/shared/model-requirements.ts

@@ -78,7 +78,7 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   librarian: {
     fallbackChain: [
       { providers: ["opencode-go"], model: "minimax-m2.7" },
-      { providers: ["opencode"], model: "minimax-m2.5" },
+      { providers: ["opencode"], model: "minimax-m2.7-highspeed" },
       { providers: ["anthropic", "opencode"], model: "claude-haiku-4-5" },
       { providers: ["opencode"], model: "gpt-5-nano" },
     ],
@@ -86,8 +86,8 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   explore: {
     fallbackChain: [
       { providers: ["github-copilot", "xai"], model: "grok-code-fast-1" },
-      { providers: ["opencode-go"], model: "minimax-m2.7" },
-      { providers: ["opencode"], model: "minimax-m2.5" },
+      { providers: ["opencode-go"], model: "minimax-m2.7-highspeed" },
+      { providers: ["opencode"], model: "minimax-m2.7" },
       { providers: ["anthropic", "opencode"], model: "claude-haiku-4-5" },
       { providers: ["opencode"], model: "gpt-5-nano" },
     ],