feat(tui): add search via capture bar and absorb flow

Search uses existing parse grammar ?prefix — type `?query #tag` in
capture bar to filter entities client-side. Substring match on
body+title+description with AND tag filtering. Esc clears search.

Absorb via m key on fluid entities — opens source picker showing all
other entities, enter merges source into target. Uses existing
store.Absorb() backend.

internal/tui/search.go

@@ -0,0 +1,55 @@
+package tui
+
+import (
+	"strings"
+
+	"github.com/lerko/nib/internal/db"
+)
+
+func filterEntities(entities []*db.Entity, query string, tags []string) []*db.Entity {
+	if query == "" && len(tags) == 0 {
+		return entities
+	}
+
+	query = strings.ToLower(query)
+	lowerTags := make([]string, len(tags))
+	for i, t := range tags {
+		lowerTags[i] = strings.ToLower(t)
+	}
+
+	var result []*db.Entity
+	for _, e := range entities {
+		if !matchesSearch(e, query, lowerTags) {
+			continue
+		}
+		result = append(result, e)
+	}
+	return result
+}
+
+func matchesSearch(e *db.Entity, query string, tags []string) bool {
+	if len(tags) > 0 {
+		eTags := make(map[string]bool, len(e.Tags))
+		for _, t := range e.Tags {
+			eTags[strings.ToLower(t)] = true
+		}
+		for _, t := range tags {
+			if !eTags[t] {
+				return false
+			}
+		}
+	}
+
+	if query == "" {
+		return true
+	}
+
+	haystack := strings.ToLower(e.Body)
+	if e.Title != nil {
+		haystack += " " + strings.ToLower(*e.Title)
+	}
+	if e.Description != nil {
+		haystack += " " + strings.ToLower(*e.Description)
+	}
+	return strings.Contains(haystack, query)
+}