feat(pi): emit model context and output limits when set

The pi adapter writes contextWindow and maxTokens to models.json only
when the profile sets them. Pi keeps its own defaults for every other
model. Tests cover both cases.

internal/adapters/pi/pi_test.go

@@ -56,6 +56,41 @@ func TestPiRender(t *testing.T) {
 	first := modelList[0].(map[string]any)
 	assert.Equal(t, "claude-sonnet-4-6", first["id"])
 	assert.Equal(t, "Sonnet", first["name"])
+	// A model with no explicit limits must carry none, so pi keeps applying
+	// its own defaults rather than inheriting canonical's.
+	assert.NotContains(t, first, "contextWindow")
+	assert.NotContains(t, first, "maxTokens")
+}
+
+func TestPiRenderEmitsExplicitModelLimits(t *testing.T) {
+	home := t.TempDir()
+	ad := New(WithHome(home))
+	b := &canonical.Bundle{
+		Profile: canonical.Profile{
+			Gateway: canonical.Gateway{URL: "https://gw", Token: "dummy"},
+			Models: []canonical.Model{
+				{ID: "local/current-model", Alias: "Local", Context: 1048576, Output: 32768},
+				{ID: "bare/model"},
+			},
+		},
+	}
+	fs, err := ad.Render(b)
+	require.NoError(t, err)
+
+	seen := map[string]adapter.File{}
+	fs.ForEach(func(f adapter.File) { seen[f.Dest] = f })
+
+	var models map[string]any
+	require.NoError(t, json.Unmarshal(seen[filepath.Join(home, ".pi", "agent", "models.json")].Content, &models))
+	list := models["providers"].(map[string]any)["hs-gw"].(map[string]any)["models"].([]any)
+
+	withLimits := list[0].(map[string]any)
+	assert.InDelta(t, float64(1048576), withLimits["contextWindow"], 0)
+	assert.InDelta(t, float64(32768), withLimits["maxTokens"], 0)
+
+	withoutLimits := list[1].(map[string]any)
+	assert.NotContains(t, withoutLimits, "contextWindow")
+	assert.NotContains(t, withoutLimits, "maxTokens")
 }
 
 func TestPiRenderMergesExistingSettings(t *testing.T) {