Refactor CLI: birdc-style help, collapsed nouns, ReloadConfig, bug fixes

maglevc
- Rewrite '?' handler (birdc-style): show full command paths from current
  position to every leaf, right-aligned help column, dynamic slot values
  displayed as an indented block when cursor is at a slot position.
- Collapse show frontends/frontend, backends/backend, healthchecks/healthcheck
  into single plural-noun nodes with an optional <name> slot. Allows
  'sh ba' (list all) and 'sh ba nginx0' (show one) without ambiguity.
- Add 'config reload' command.
- Fix tabwriter ANSI alignment: continuation lines in transition output
  now carry the same label() byte overhead as the header line.
- Fix broken Walk for 'set frontend' command: setFrontendPoolName and
  setWeightValue were fixed-word nodes that couldn't capture user input;
  mark them as slot nodes with dynNone.
- Add tree_test.go covering expandPaths, cycle detection, prefix matching,
  and the full weight-command walk.

gRPC / proto
- Add ReloadConfig RPC: checks config then applies it to the running
  checker, returning ok/parse_error/semantic_error/reload_error.
- Add logging to CheckConfig (config-check-start/config-check-done at
  INFO level).

maglevd
- SIGHUP handler now calls maglevServer.TriggerReload(), sharing the
  same code path as the gRPC ReloadConfig RPC.

docs
- Collapse show command documentation to use [<name>] optional syntax.
- Remove developer-facing 'Command tree and parser' section.
- Document 'config reload'.

internal/grpcapi/maglev_grpc.pb.go

@@ -32,6 +32,7 @@ const (
 	Maglev_SetFrontendPoolBackendWeight_FullMethodName = "/maglev.Maglev/SetFrontendPoolBackendWeight"
 	Maglev_WatchEvents_FullMethodName                  = "/maglev.Maglev/WatchEvents"
 	Maglev_CheckConfig_FullMethodName                  = "/maglev.Maglev/CheckConfig"
+	Maglev_ReloadConfig_FullMethodName                 = "/maglev.Maglev/ReloadConfig"
 )
 
 // MaglevClient is the client API for Maglev service.
@@ -53,6 +54,7 @@ type MaglevClient interface {
 	SetFrontendPoolBackendWeight(ctx context.Context, in *SetWeightRequest, opts ...grpc.CallOption) (*FrontendInfo, error)
 	WatchEvents(ctx context.Context, in *WatchRequest, opts ...grpc.CallOption) (grpc.ServerStreamingClient[Event], error)
 	CheckConfig(ctx context.Context, in *CheckConfigRequest, opts ...grpc.CallOption) (*CheckConfigResponse, error)
+	ReloadConfig(ctx context.Context, in *ReloadConfigRequest, opts ...grpc.CallOption) (*ReloadConfigResponse, error)
 }
 
 type maglevClient struct {
@@ -202,6 +204,16 @@ func (c *maglevClient) CheckConfig(ctx context.Context, in *CheckConfigRequest,
 	return out, nil
 }
 
+func (c *maglevClient) ReloadConfig(ctx context.Context, in *ReloadConfigRequest, opts ...grpc.CallOption) (*ReloadConfigResponse, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(ReloadConfigResponse)
+	err := c.cc.Invoke(ctx, Maglev_ReloadConfig_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
 // MaglevServer is the server API for Maglev service.
 // All implementations must embed UnimplementedMaglevServer
 // for forward compatibility.
@@ -221,6 +233,7 @@ type MaglevServer interface {
 	SetFrontendPoolBackendWeight(context.Context, *SetWeightRequest) (*FrontendInfo, error)
 	WatchEvents(*WatchRequest, grpc.ServerStreamingServer[Event]) error
 	CheckConfig(context.Context, *CheckConfigRequest) (*CheckConfigResponse, error)
+	ReloadConfig(context.Context, *ReloadConfigRequest) (*ReloadConfigResponse, error)
 	mustEmbedUnimplementedMaglevServer()
 }
 
@@ -270,6 +283,9 @@ func (UnimplementedMaglevServer) WatchEvents(*WatchRequest, grpc.ServerStreaming
 func (UnimplementedMaglevServer) CheckConfig(context.Context, *CheckConfigRequest) (*CheckConfigResponse, error) {
 	return nil, status.Error(codes.Unimplemented, "method CheckConfig not implemented")
 }
+func (UnimplementedMaglevServer) ReloadConfig(context.Context, *ReloadConfigRequest) (*ReloadConfigResponse, error) {
+	return nil, status.Error(codes.Unimplemented, "method ReloadConfig not implemented")
+}
 func (UnimplementedMaglevServer) mustEmbedUnimplementedMaglevServer() {}
 func (UnimplementedMaglevServer) testEmbeddedByValue()                {}
 
@@ -518,6 +534,24 @@ func _Maglev_CheckConfig_Handler(srv interface{}, ctx context.Context, dec func(
 	return interceptor(ctx, in, info, handler)
 }
 
+func _Maglev_ReloadConfig_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(ReloadConfigRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(MaglevServer).ReloadConfig(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: Maglev_ReloadConfig_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(MaglevServer).ReloadConfig(ctx, req.(*ReloadConfigRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
 // Maglev_ServiceDesc is the grpc.ServiceDesc for Maglev service.
 // It's only intended for direct use with grpc.RegisterService,
 // and not to be introspected or modified (even as a copy)
@@ -573,6 +607,10 @@ var Maglev_ServiceDesc = grpc.ServiceDesc{
 			MethodName: "CheckConfig",
 			Handler:    _Maglev_CheckConfig_Handler,
 		},
+		{
+			MethodName: "ReloadConfig",
+			Handler:    _Maglev_ReloadConfig_Handler,
+		},
 	},
 	Streams: []grpc.StreamDesc{
 		{