feat: Add --claim flag to bd update for work queue semantics (gt-il2p7)

Adds atomic claim operation for work queue messages:

- New --claim flag on bd update command
- Sets assignee to claimer and status to in_progress
- Fails with clear error if already claimed by someone else
- Works in both daemon and direct modes
- Includes comprehensive tests for claim functionality

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/rpc/protocol.go

@@ -152,6 +152,8 @@ type UpdateArgs struct {
 	EventActor    *string `json:"event_actor,omitempty"`    // Entity URI who caused this event
 	EventTarget   *string `json:"event_target,omitempty"`   // Entity URI or bead ID affected
 	EventPayload  *string `json:"event_payload,omitempty"`  // Event-specific JSON data
+	// Work queue claim operation
+	Claim bool `json:"claim,omitempty"` // If true, atomically claim issue (set assignee+status, fail if already claimed)
 }
 
 // CloseArgs represents arguments for the close operation