feat: add absorb command — merge source entity into target

DB: Absorb() merges body (newline-separated), unions tags, demotes
crystallized sources, soft-deletes source. Rejects crystallized targets.

API: POST /api/entities/:id/absorb { source_id }

CLI: nib absorb <target> <source> with prefix ID resolution

Web: absorb button on fluid entities, 'a' keyboard shortcut,
source picker modal

cmd/absorb.go

@@ -0,0 +1,53 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/lerko/nib/internal/db"
+	"github.com/lerko/nib/internal/display"
+	"github.com/spf13/cobra"
+)
+
+var absorbCmd = &cobra.Command{
+	Use:   "absorb <target> <source>",
+	Short: "pull source material into target, ghost the source",
+	Args:  cobra.ExactArgs(2),
+	RunE:  runAbsorb,
+}
+
+func init() {
+	rootCmd.AddCommand(absorbCmd)
+}
+
+func runAbsorb(_ *cobra.Command, args []string) error {
+	store, err := openStore()
+	if err != nil {
+		return err
+	}
+	defer store.Close()
+
+	targetID, err := store.Resolve(args[0])
+	if err != nil {
+		return fmt.Errorf("not_found — no entity with id %s", args[0])
+	}
+
+	sourceID, err := store.Resolve(args[1])
+	if err != nil {
+		return fmt.Errorf("not_found — no entity with id %s", args[1])
+	}
+
+	if targetID == sourceID {
+		return fmt.Errorf("target and source must be different entities")
+	}
+
+	if err := store.Absorb(targetID, sourceID); err != nil {
+		if err == db.ErrTargetCrystallized {
+			return fmt.Errorf("invalid_absorb — target %s is crystallized, demote first",
+				display.FormatID(targetID))
+		}
+		return err
+	}
+
+	fmt.Printf("absorbed %s → %s\n", display.FormatID(sourceID), display.FormatID(targetID))
+	return nil
+}