feat: add --estimate flag to bd create/update commands (GH #443)

The estimated_minutes field existed in the Issue schema but wasn't exposed
via CLI. This adds:

- --estimate / -e flag to bd create (e.g., bd create "Task" --estimate 120)
- --estimate / -e flag to bd update (e.g., bd update bd-xyz --estimate 60)
- EstimatedMinutes field to RPC CreateArgs and UpdateArgs
- Server-side handling in handleCreate and updatesFromArgs
- Validation for non-negative values

The value is specified in minutes and is useful for planning and
prioritization. The vscode-beads extension already has an Estimate column
that can now be populated.

Fixes #443

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

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

internal/rpc/server_issues_epics.go

@@ -70,6 +70,9 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.ExternalRef != nil {
 		u["external_ref"] = *a.ExternalRef
 	}
+	if a.EstimatedMinutes != nil {
+		u["estimated_minutes"] = *a.EstimatedMinutes
+	}
 	return u
 }
 
@@ -142,6 +145,7 @@ func (s *Server) handleCreate(req *Request) Response {
 		AcceptanceCriteria: strValue(acceptance),
 		Assignee:           strValue(assignee),
 		ExternalRef:        externalRef,
+		EstimatedMinutes:   createArgs.EstimatedMinutes,
 		Status:             types.StatusOpen,
 	}