feat: activity fuzzy matching (exact > prefix > substring)

@@ -0,0 +1,41 @@
+package day
+
+import "strings"
+
+// ActivityType is the CLI-side view of a Dots activity type.
+type ActivityType struct {
+	ID    string
+	Name  string
+	Color string
+}
+
+// MatchActivity finds candidate types for a query: exact (case-insensitive)
+// wins outright; if multiple names match exactly, the first wins; otherwise all
+// prefix matches; otherwise all substring matches. Empty or whitespace-only queries
+// return no results.
+func MatchActivity(query string, types []ActivityType) []ActivityType {
+	q := strings.TrimSpace(query)
+	if q == "" {
+		return []ActivityType{}
+	}
+	q = strings.ToLower(q)
+	var prefix, substr []ActivityType
+	for _, t := range types {
+		n := strings.ToLower(t.Name)
+		if n == q {
+			return []ActivityType{t}
+		}
+		if strings.HasPrefix(n, q) {
+			prefix = append(prefix, t)
+		} else if strings.Contains(n, q) {
+			substr = append(substr, t)
+		}
+	}
+	if len(prefix) > 0 {
+		return prefix
+	}
+	if substr == nil {
+		return []ActivityType{}
+	}
+	return substr
+}

@@ -0,0 +1,69 @@
+package day
+
+import "testing"
+
+var types = []ActivityType{
+	{ID: "1", Name: "ARCHER", Color: "#c04000"},
+	{ID: "2", Name: "Archery practice", Color: "#00c040"},
+	{ID: "3", Name: "Foofaraw", Color: "#4000c0"},
+	{ID: "4", Name: "Reading", Color: "#404040"},
+}
+
+var typesWithDuplicates = []ActivityType{
+	{ID: "4a", Name: "Reading", Color: "#404040"},
+	{ID: "4b", Name: "READING", Color: "#505050"},
+}
+
+func names(ms []ActivityType) []string {
+	out := make([]string, len(ms))
+	for i, m := range ms {
+		out[i] = m.Name
+	}
+	return out
+}
+
+func ids(ms []ActivityType) []string {
+	out := make([]string, len(ms))
+	for i, m := range ms {
+		out[i] = m.ID
+	}
+	return out
+}
+
+func TestMatchActivity(t *testing.T) {
+	cases := []struct {
+		query string
+		want  []string
+	}{
+		{"archer", []string{"ARCHER"}},          // exact beats prefix
+		{"arch", []string{"ARCHER", "Archery practice"}}, // prefix, both
+		{"foo", []string{"Foofaraw"}},
+		{"ading", []string{"Reading"}},          // substring fallback
+		{"xyz", []string{}},
+		{"READING", []string{"Reading"}},
+		{"", []string{}},                        // empty query
+		{"  ", []string{}},                      // whitespace-only query
+	}
+	for _, c := range cases {
+		got := names(MatchActivity(c.query, types))
+		if len(got) != len(c.want) {
+			t.Fatalf("%q: got %v, want %v", c.query, got, c.want)
+		}
+		for i := range got {
+			if got[i] != c.want[i] {
+				t.Fatalf("%q: got %v, want %v", c.query, got, c.want)
+			}
+		}
+	}
+}
+
+func TestMatchActivityExactTies(t *testing.T) {
+	// When multiple names match exactly (case-insensitive), first wins
+	got := ids(MatchActivity("reading", typesWithDuplicates))
+	if len(got) != 1 {
+		t.Fatalf("reading: got %d results, want 1", len(got))
+	}
+	if got[0] != "4a" {
+		t.Fatalf("reading: got ID %q, want %q (first match)", got[0], "4a")
+	}
+}