Add daemon fallback visibility and version compatibility checks

Implemented bd-150: Improve daemon fallback visibility and user feedback
- Added DaemonStatus struct to track connection state
- Enhanced BD_DEBUG logging with detailed diagnostics and timing
- Added BD_VERBOSE mode with actionable warnings when falling back
- Implemented health checks before using daemon
- Clear fallback reasons: connect_failed, health_failed, auto_start_disabled, auto_start_failed, flag_no_daemon
- Updated documentation

Implemented bd-151: Add version compatibility checks for daemon RPC protocol
- Added ClientVersion field to RPC Request struct
- Client sends version (0.9.10) in all requests
- Server validates version compatibility using semver:
  - Major version must match
  - Daemon >= client for backward compatibility
  - Clear error messages with directional hints (upgrade daemon vs upgrade client)
- Added ClientVersion and Compatible fields to HealthResponse
- Implemented 'bd version --daemon' command to check compatibility
- Fixed batch operations to propagate ClientVersion for proper checks
- Updated documentation with version compatibility section

Code review improvements:
- Propagate ClientVersion in batch sub-requests
- Directional error messages based on which side is older
- Made ServerVersion a var for future unification

Amp-Thread-ID: https://ampcode.com/threads/T-b5fe36b8-c065-44a9-a55b-582573671609
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/protocol.go

@@ -31,11 +31,12 @@ const (
 
 // Request represents an RPC request from client to daemon
 type Request struct {
-	Operation string          `json:"operation"`
-	Args      json.RawMessage `json:"args"`
-	Actor     string          `json:"actor,omitempty"`
-	RequestID string          `json:"request_id,omitempty"`
-	Cwd       string          `json:"cwd,omitempty"` // Working directory for database discovery
+	Operation     string          `json:"operation"`
+	Args          json.RawMessage `json:"args"`
+	Actor         string          `json:"actor,omitempty"`
+	RequestID     string          `json:"request_id,omitempty"`
+	Cwd           string          `json:"cwd,omitempty"`      // Working directory for database discovery
+	ClientVersion string          `json:"client_version,omitempty"` // Client version for compatibility checks
 }
 
 // Response represents an RPC response from daemon to client
@@ -141,7 +142,9 @@ type PingResponse struct {
 // HealthResponse is the response for a health check operation
 type HealthResponse struct {
 	Status          string  `json:"status"`           // "healthy", "degraded", "unhealthy"
-	Version         string  `json:"version"`
+	Version         string  `json:"version"`          // Server/daemon version
+	ClientVersion   string  `json:"client_version,omitempty"`  // Client version from request
+	Compatible      bool    `json:"compatible"`       // Whether versions are compatible
 	Uptime          float64 `json:"uptime_seconds"`
 	CacheSize       int     `json:"cache_size"`
 	CacheHits       int64   `json:"cache_hits"`