fix: Prevent cross-family model matching in resolveModelPreference (#731)

## Summary

Partially addresses #639. Builds on #702 which added version-aware
matching.

When the SDK's `opus` alias resolves to a newer version than what's in
`availableModels` (e.g. the SDK was bumped to Opus 4.8 but
`availableModels` still contains `claude-opus-4-6[1m]`), the version
check from #702 correctly rejects the `opus` entry (4.8 ≠ 4.6). However,
the tokenized matcher then falls through to `sonnet[1m]` because the
`1m` context hint token alone scores 3 points — enough to win. This
causes `claude-opus-4-6[1m]` to display as "Sonnet (1M context)".

Fix: `scoreModelMatch` now returns 0 when the only matching token is the
context hint. This prevents cross-family matches without hardcoding
model family names.

Note: Claude initially recommended hardcoding model families to be 100%
sure we avoid cross-family matching. While it would be a stronger
signal, I didn't find references to model families in the codebase
(except for opusplan --> opus) so I'm assuming the family list is
transparent to claude-agent-acp.

## Test plan

- [x] Added test: `does not inherit display info across mismatched model
families` — verifies `claude-opus-4-6[1m]` does not resolve to `Sonnet
(1M context)`
- [x] Existing tests pass (20/20 in `acp-agent-settings.test.ts`)
- [x] New test confirmed to fail on unmodified `main` and pass with the
fix

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/acp-agent.ts

@@ -2739,11 +2739,14 @@ function tokenizeModelPreference(model: string): { tokens: string[]; contextHint
 function scoreModelMatch(model: ModelInfo, tokens: string[], contextHint?: string): number {
   const haystack = `${model.value} ${model.displayName}`.toLowerCase();
   let score = 0;
+  let nonHintMatched = false;
   for (const token of tokens) {
     if (haystack.includes(token)) {
+      if (token !== contextHint) nonHintMatched = true;
       score += token === contextHint ? 3 : 1;
     }
   }
+  if (contextHint && !nonHintMatched) return 0;
   return score;
 }
 

src/tests/acp-agent-settings.test.ts

@@ -585,7 +585,7 @@ describe("ClaudeAcpAgent settings", () => {
       );
     });
 
-    it("does not inherit display info across mismatched model family versions", async () => {
+    it("does not inherit display info across mismatched model versions", async () => {
       // https://github.com/agentclientprotocol/claude-agent-acp/issues/639:
       // when the SDK's `opus` alias resolves to Opus 4.7, an allowlist entry
       // of `claude-opus-4-6` (or `claude-opus-4-6[1m]`) used to substring-match
@@ -646,6 +646,60 @@ describe("ClaudeAcpAgent settings", () => {
       expect(byValue["claude-opus-4-7"].name).toBe("Opus 4.7");
       expect(byValue["claude-opus-4-7[1m]"].name).toMatch(/Opus 4\.7/);
     });
+
+    it("does not inherit display info across mismatched model families", async () => {
+      // https://github.com/agentclientprotocol/claude-agent-acp/issues/639:
+      // when the SDK's `opus` alias resolves to Opus 4.8, `claude-opus-4-6[1m]`
+      // fails the version check against `opus` but the tokenized matcher
+      // falls through to `sonnet[1m]` because the `1m` context hint alone
+      // is enough to score a match.
+      await fs.promises.writeFile(
+        path.join(tempDir, "settings.json"),
+        JSON.stringify({
+          availableModels: ["claude-opus-4-6", "claude-opus-4-6[1m]"],
+        }),
+      );
+
+      const projectDir = path.join(tempDir, "project");
+      await fs.promises.mkdir(projectDir, { recursive: true });
+
+      mockQueryWithModels([
+        { value: "default", displayName: "Default", description: "Default model" },
+        {
+          value: "sonnet",
+          displayName: "Sonnet",
+          description: "Sonnet 4.6 · Best for everyday tasks",
+        },
+        {
+          value: "sonnet[1m]",
+          displayName: "Sonnet (1M context)",
+          description: "Sonnet 4.6 for long sessions",
+        },
+        {
+          value: "opus",
+          displayName: "Opus 4.8",
+          description: "Claude Opus 4.8",
+        },
+      ]);
+
+      const { ClaudeAcpAgent } = await import("../acp-agent.js");
+      const agent: ClaudeAcpAgentType = new ClaudeAcpAgent(createMockClient());
+
+      const response = await (agent as any).createSession({
+        cwd: projectDir,
+        mcpServers: [],
+        _meta: { disableBuiltInTools: true },
+      });
+
+      const modelOption = response.configOptions.find((o: any) => o.id === "model");
+      const byValue: Record<string, { name: string; description?: string }> = {};
+      for (const opt of modelOption.options) {
+        byValue[opt.value] = { name: opt.name, description: opt.description };
+      }
+
+      expect(byValue["claude-opus-4-6[1m]"].name).toBe("claude-opus-4-6[1m]");
+      expect(byValue["claude-opus-4-6"].name).toBe("claude-opus-4-6");
+    });
   });
 
   it("resolves model aliases like opus[1m] to the correct model", async () => {