Implement daemon RPC with per-request context routing (bd-115)

- Added per-request storage routing in daemon server
  - Server now supports Cwd field in requests for database discovery
  - Tree-walking to find .beads/*.db from any working directory
  - Storage caching for performance across requests

- Created Python daemon client (bd_daemon_client.py)
  - RPC over Unix socket communication
  - Implements full BdClientBase interface
  - Auto-discovery of daemon socket from working directory

- Refactored bd_client.py with abstract interface
  - BdClientBase abstract class for common interface
  - BdCliClient for CLI-based operations (renamed from BdClient)
  - create_bd_client() factory with daemon/CLI fallback
  - Backwards-compatible BdClient alias

Next: Update MCP server to use daemon client when available

internal/rpc/protocol.go

@@ -26,6 +26,7 @@ type Request struct {
 	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
 }
 
 // Response represents an RPC response from daemon to client