Add configurable sort policy for GetReadyWork (bd-147)

- Add SortPolicy type with hybrid, priority, oldest constants
- Add SortPolicy field to WorkFilter
- Implement buildOrderByClause() for SQL generation
- Add --sort flag to bd ready command
- Add comprehensive tests for all 3 sort policies
- Update RPC protocol to support sort policy
- Update documentation with sort policy examples

Enables autonomous systems like VC to use strict priority ordering
while preserving hybrid behavior for interactive use.

Amp-Thread-ID: https://ampcode.com/threads/T-9d7ea9db-8d6d-4498-9daa-48a7e104ce1f
Co-authored-by: Amp <amp@ampcode.com>

internal/rpc/server.go

@@ -1177,9 +1177,10 @@ func (s *Server) handleReady(req *Request) Response {
 	}
 
 	wf := types.WorkFilter{
-		Status:   types.StatusOpen,
-		Priority: readyArgs.Priority,
-		Limit:    readyArgs.Limit,
+		Status:     types.StatusOpen,
+		Priority:   readyArgs.Priority,
+		Limit:      readyArgs.Limit,
+		SortPolicy: types.SortPolicy(readyArgs.SortPolicy),
 	}
 	if readyArgs.Assignee != "" {
 		wf.Assignee = &readyArgs.Assignee