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

internal/db/db.go

@@ -10,9 +10,10 @@ import (
 )
 
 var (
-	ErrNotFound        = errors.New("not_found")
-	ErrAlreadyPromoted = errors.New("invalid_promote")
-	ErrAlreadyFluid    = errors.New("invalid_demote")
+	ErrNotFound           = errors.New("not_found")
+	ErrAlreadyPromoted    = errors.New("invalid_promote")
+	ErrAlreadyFluid       = errors.New("invalid_demote")
+	ErrTargetCrystallized = errors.New("invalid_absorb")
 )
 
 type Store struct {