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'.

cmd/maglevd/main.go

@@ -93,7 +93,8 @@ func run() error {
 		return fmt.Errorf("listen %s: %w", *grpcAddr, err)
 	}
 	srv := grpc.NewServer()
-	grpcapi.RegisterMaglevServer(srv, grpcapi.NewServer(ctx, chkr, logBroadcaster, *configPath))
+	maglevServer := grpcapi.NewServer(ctx, chkr, logBroadcaster, *configPath)
+	grpcapi.RegisterMaglevServer(srv, maglevServer)
 	if *enableReflection {
 		reflection.Register(srv)
 	}
@@ -112,21 +113,7 @@ func run() error {
 	for sig := range sigCh {
 		switch sig {
 		case syscall.SIGHUP:
-			slog.Info("config-reload-start")
-			newCfg, result := config.Check(*configPath)
-			if !result.OK() {
-				if result.ParseError != "" {
-					slog.Error("config-check-failed", "type", "parse", "err", result.ParseError)
-				} else {
-					slog.Error("config-check-failed", "type", "semantic", "err", result.SemanticError)
-				}
-				continue
-			}
-			if err := chkr.Reload(ctx, newCfg); err != nil {
-				slog.Error("checker-reload-error", "err", err)
-				continue
-			}
-			slog.Info("config-reload-done", "frontends", len(newCfg.Frontends))
+			maglevServer.TriggerReload()
 
 		case syscall.SIGTERM, syscall.SIGINT:
 			slog.Info("shutdown", "signal", sig)