feat(tui): add tag autocomplete and query composition

Tag autocomplete shows suggestions when typing #partial in capture bar.
Tab/enter accepts, up/down navigates, esc dismisses.

Query composition extends ? search with date filters (@today, @week,
@month, <7d, >30d), card type filters (^snippet), all composable
with existing text and tag filters.

internal/tui/autocomplete_test.go

@@ -0,0 +1,85 @@
+package tui
+
+import (
+	"testing"
+
+	"github.com/lerko/nib/internal/db"
+)
+
+func TestTagTokenAtCursor(t *testing.T) {
+	tests := []struct {
+		name      string
+		val       string
+		cursor    int
+		wantStart int
+		wantEnd   int
+		wantPfx   string
+		wantOk    bool
+	}{
+		{"mid tag cursor after a", "hello #par world", 9, 6, 10, "pa", true},
+		{"end of tag", "hello #par world", 10, 6, 10, "par", true},
+		{"end of input", "hello #parenting", 16, 6, 16, "parenting", true},
+		{"start of tag just hash", "hello # world", 7, 6, 7, "", true},
+		{"not in tag", "hello world", 5, 0, 0, "", false},
+		{"tag at start", "#ops stuff", 4, 0, 4, "ops", true},
+		{"cursor at hash", "#ops", 1, 0, 4, "", true},
+		{"multiple tags second", "hello #ops #inf", 15, 11, 15, "inf", true},
+		{"empty string", "", 0, 0, 0, "", false},
+		{"cursor past end", "#ops", 10, 0, 4, "ops", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			start, end, pfx, ok := tagTokenAtCursor(tt.val, tt.cursor)
+			if ok != tt.wantOk {
+				t.Fatalf("ok = %v, want %v", ok, tt.wantOk)
+			}
+			if !ok {
+				return
+			}
+			if start != tt.wantStart || end != tt.wantEnd {
+				t.Fatalf("range = [%d,%d), want [%d,%d)", start, end, tt.wantStart, tt.wantEnd)
+			}
+			if pfx != tt.wantPfx {
+				t.Fatalf("prefix = %q, want %q", pfx, tt.wantPfx)
+			}
+		})
+	}
+}
+
+func TestFilterTagSuggestions(t *testing.T) {
+	tags := []db.TagCount{
+		{Tag: "ops", Count: 5},
+		{Tag: "ops-deploy", Count: 3},
+		{Tag: "infra", Count: 2},
+		{Tag: "ops-team", Count: 1},
+	}
+
+	tests := []struct {
+		name   string
+		prefix string
+		want   []string
+	}{
+		{"empty prefix", "", nil},
+		{"exact match excluded", "ops", []string{"ops-deploy", "ops-team"}},
+		{"partial match", "op", []string{"ops", "ops-deploy", "ops-team"}},
+		{"no match", "zzz", nil},
+		{"case insensitive", "OP", []string{"ops", "ops-deploy", "ops-team"}},
+		{"single match", "inf", []string{"infra"}},
+		{"full match excluded", "infra", nil},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := filterTagSuggestions(tags, tt.prefix)
+			if len(got) != len(tt.want) {
+				t.Fatalf("got %v, want %v", got, tt.want)
+			}
+			for i := range got {
+				if got[i] != tt.want[i] {
+					t.Fatalf("got %v, want %v", got, tt.want)
+				}
+			}
+		})
+	}
+}