Add daemon health check endpoint (bd-146)

- Add OpHealth RPC operation to protocol
- Implement handleHealth() with DB ping and 1s timeout
- Returns status (healthy/degraded/unhealthy), uptime, cache metrics
- Update TryConnect() to use health check instead of ping
- Add 'bd daemon --health' CLI command with JSON output
- Track cache hits/misses for metrics
- Unhealthy daemon triggers automatic fallback to direct mode
- Health check completes in <2 seconds

Amp-Thread-ID: https://ampcode.com/threads/T-1a4889f3-77cf-433a-a704-e1c383929f48
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/protocol.go

@@ -9,6 +9,7 @@ import (
 // Operation constants for all bd commands
 const (
 	OpPing           = "ping"
+	OpHealth         = "health"
 	OpCreate         = "create"
 	OpUpdate         = "update"
 	OpClose          = "close"
@@ -137,6 +138,18 @@ type PingResponse struct {
 	Version string `json:"version"`
 }
 
+// HealthResponse is the response for a health check operation
+type HealthResponse struct {
+	Status          string  `json:"status"`           // "healthy", "degraded", "unhealthy"
+	Version         string  `json:"version"`
+	Uptime          float64 `json:"uptime_seconds"`
+	CacheSize       int     `json:"cache_size"`
+	CacheHits       int64   `json:"cache_hits"`
+	CacheMisses     int64   `json:"cache_misses"`
+	DBResponseTime  float64 `json:"db_response_ms"`
+	Error           string  `json:"error,omitempty"`
+}
+
 // BatchArgs represents arguments for batch operations
 type BatchArgs struct {
 	Operations []BatchOperation `json:"operations"`