Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

backend/internal/model/note.go

@@ -0,0 +1,51 @@
+package model
+
+// Note represents a note attached to a contact.
+// Reference: Chatwoot app/models/note.rb
+//
+// Table name: notes
+//
+//  id         uint   primaryKey autoIncrement
+//  content    string not null
+//  account_id uint   not null  gorm:"index"
+//  contact_id uint   not null  gorm:"index"
+//  user_id    *uint  gorm:"index"  (optional — nullable)
+//  created_at time
+//  updated_at time
+//  deleted_at gorm.DeletedAt gorm:"index"
+
+import (
+	"gorm.io/gorm"
+)
+
+// Note is a text note attached to a contact, similar to Chatwoot's Note model.
+// Notes allow agents to add internal comments/observations about a contact.
+// Content is required; account_id and contact_id are required; user_id is optional.
+type Note struct {
+	Base
+	Content   string `gorm:"type:text;not null" json:"content" binding:"required"`
+	AccountID uint   `gorm:"not null;index" json:"account_id"`
+	ContactID uint   `gorm:"not null;index" json:"contact_id"`
+	UserID    *uint  `gorm:"index" json:"user_id,omitempty"`
+	User      *User  `gorm:"foreignKey:UserID" json:"user,omitempty"`
+
+	Account  Account `gorm:"foreignKey:AccountID" json:"-"`
+	Contact  Contact `gorm:"foreignKey:ContactID" json:"-"`
+}
+
+// TableName returns the database table name for Note.
+func (Note) TableName() string {
+	return "notes"
+}
+
+// BeforeCreate hook: ensure account_id is set from contact's account_id if missing.
+// Reference: Chatwoot's `before_validation :ensure_account_id`
+func (n *Note) BeforeCreate(db *gorm.DB) error {
+	if n.AccountID == 0 && n.ContactID != 0 {
+		var contact Contact
+		if err := db.First(&contact, n.ContactID).Error; err == nil {
+			n.AccountID = contact.AccountID
+		}
+	}
+	return nil
+}