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/copilot_thread.go.txt

@@ -0,0 +1,30 @@
+package model
+
+import (
+	"encoding/json"
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// CopilotThread represents an AI copilot conversation thread.
+// Reference: P2B M10 spec
+type CopilotThread struct {
+	ID          uint           `gorm:"primaryKey" json:"id"`
+	AccountID   uint           `gorm:"not null;index" json:"account_id"`
+	UserID      uint           `gorm:"not null;index" json:"user_id"`
+	ConversationID *uint       `gorm:"index" json:"conversation_id,omitempty"`
+	Title       string         `gorm:"size:255" json:"title"`
+	Status      string         `gorm:"size:50;default:'active'" json:"status"` // active/archived
+	Metadata    json.RawMessage `gorm:"type:jsonb" json:"metadata"`
+	CreatedAt   time.Time      `gorm:"autoCreateTime" json:"created_at"`
+	UpdatedAt   time.Time      `gorm:"autoUpdateTime" json:"updated_at"`
+	DeletedAt   gorm.DeletedAt `gorm:"index" json:"deleted_at,omitempty"`
+
+	Account      Account         `gorm:"foreignKey:AccountID" json:"account,omitempty"`
+	User         User            `gorm:"foreignKey:UserID" json:"user,omitempty"`
+	Conversation *Conversation   `gorm:"foreignKey:ConversationID" json:"conversation,omitempty"`
+	Messages     []CopilotMessage `gorm:"foreignKey:ThreadID" json:"messages,omitempty"`
+}
+
+func (CopilotThread) TableName() string { return "copilot_threads" }