Implement BrainRegistry — one connection manager per robot ESN

[infraax]

Task: BrainRegistry

Parent epic: #4 (M1 — Robot Connection)
Depends on: #14 (vector-go-sdk in go.mod)

What to do

Create internal/brain/registry.go — a registry that holds one ConnectionManager per robot ESN. The Go HTTP handlers get/create robot connections through this registry.

// Package brain manages persistent SDK connections to Vector robots.
// One ConnectionManager per ESN, created on first event and held for the process lifetime.
package brain

type Registry struct { ... }

func NewRegistry(cfg *config.Config) *Registry
func (r *Registry) Get(esn string) (*ConnectionManager, bool)
func (r *Registry) GetOrCreate(esn string) *ConnectionManager
func (r *Registry) All() []*ConnectionManager
func (r *Registry) Shutdown(ctx context.Context) error

Integration

• Registry is created in main.go and passed to buildRouter
• /health reads registry.All() to report connection status
• /api/v1/robots reads registry.All() for the robot list
• /api/v1/robots/{esn}/state reads registry.Get(esn) for state

Notes

• One registry, held for process lifetime. Thread-safe (multiple HTTP handlers read it concurrently).
• The registry does not create connections proactively — ConnectionManager does that lazily on first use or immediately if configured.

Definition of done

Registry compiles. /health returns "connected" or "disconnected" per ESN (not "configured").