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/maglevc/commands.go

@@ -27,58 +27,55 @@ func buildTree() *Node {
 	exit := &Node{Word: "exit", Help: "exit the shell", Run: runQuit}
 
 	// show version
-	showVersion := &Node{Word: "version", Help: "show build version", Run: runShowVersion}
+	showVersion := &Node{Word: "version", Help: "Show build version", Run: runShowVersion}
 
-	// show frontends
-	showFrontends := &Node{Word: "frontends", Help: "list all frontends", Run: runShowFrontends}
-	// show frontend <name>
+	// show frontends [<name>]  — without name: list all, with name: show details
 	showFrontendName := &Node{
 		Word:    "<name>",
-		Help:    "frontend name",
+		Help:    "Show details for a single frontend",
 		Dynamic: dynFrontends,
 		Run:     runShowFrontend,
 	}
-	showFrontend := &Node{
-		Word:     "frontend",
-		Help:     "show a single frontend",
+	showFrontends := &Node{
+		Word:     "frontends",
+		Help:     "List all frontends",
+		Run:      runShowFrontends,
 		Children: []*Node{showFrontendName},
 	}
 
-	// show backends
-	showBackends := &Node{Word: "backends", Help: "list all backends", Run: runShowBackends}
-	// show backend <name>
+	// show backends [<name>]  — without name: list all, with name: show details
 	showBackendName := &Node{
 		Word:    "<name>",
-		Help:    "backend name",
+		Help:    "Show details for a single backend",
 		Dynamic: dynBackends,
 		Run:     runShowBackend,
 	}
-	showBackend := &Node{
-		Word:     "backend",
-		Help:     "show a single backend",
+	showBackends := &Node{
+		Word:     "backends",
+		Help:     "List all backends",
+		Run:      runShowBackends,
 		Children: []*Node{showBackendName},
 	}
 
-	// show healthchecks
-	showHealthChecks := &Node{Word: "healthchecks", Help: "list all health checks", Run: runShowHealthChecks}
-	// show healthcheck <name>
+	// show healthchecks [<name>]  — without name: list all, with name: show details
 	showHealthCheckName := &Node{
 		Word:    "<name>",
-		Help:    "health check name",
+		Help:    "Show details for a single health check",
 		Dynamic: dynHealthChecks,
 		Run:     runShowHealthCheck,
 	}
-	showHealthCheck := &Node{
-		Word:     "healthcheck",
-		Help:     "show a single health check",
+	showHealthChecks := &Node{
+		Word:     "healthchecks",
+		Help:     "List all health checks",
+		Run:      runShowHealthChecks,
 		Children: []*Node{showHealthCheckName},
 	}
 
 	show.Children = []*Node{
 		showVersion,
-		showFrontends, showFrontend,
-		showBackends, showBackend,
-		showHealthChecks, showHealthCheck,
+		showFrontends,
+		showBackends,
+		showHealthChecks,
 	}
 
 	// set backend <name> pause|resume|disabled|enabled
@@ -99,9 +96,10 @@ func buildTree() *Node {
 	}
 	// set frontend <name> pool <pool> backend <name> weight <0-100>
 	setWeightValue := &Node{
-		Word: "<weight>",
-		Help: "weight 0-100",
-		Run:  runSetFrontendPoolBackendWeight,
+		Word:    "<weight>",
+		Help:    "Set weight of a backend in a pool (0-100)",
+		Dynamic: dynNone, // accepts any integer; no tab-completion candidates
+		Run:     runSetFrontendPoolBackendWeight,
 	}
 	setFrontendPoolBackendWeight := &Node{Word: "weight", Help: "set backend weight in pool", Children: []*Node{setWeightValue}}
 	setFrontendPoolBackendName := &Node{
@@ -114,6 +112,7 @@ func buildTree() *Node {
 	setFrontendPoolName := &Node{
 		Word:     "<pool>",
 		Help:     "pool name",
+		Dynamic:  dynNone, // pool names aren't listed via gRPC; accepts any input
 		Children: []*Node{setFrontendPoolBackend},
 	}
 	setFrontendPool := &Node{Word: "pool", Help: "select a pool", Children: []*Node{setFrontendPoolName}}
@@ -139,7 +138,7 @@ func buildTree() *Node {
 	var watchEventsOptSlot *Node
 	watchEventsOptSlot = &Node{
 		Word:    "<opt>",
-		Help:    "watch option",
+		Help:    "Stream events with options",
 		Dynamic: dynWatchEventOpts,
 		Run:     runWatchEvents,
 	}
@@ -157,12 +156,13 @@ func buildTree() *Node {
 		Children: []*Node{watchEvents},
 	}
 
-	// config check
-	configCheck := &Node{Word: "check", Help: "check configuration file", Run: runConfigCheck}
+	// config check / reload
+	configCheck := &Node{Word: "check", Help: "Check configuration file", Run: runConfigCheck}
+	configReload := &Node{Word: "reload", Help: "Check and reload configuration", Run: runConfigReload}
 	configNode := &Node{
 		Word:     "config",
 		Help:     "configuration commands",
-		Children: []*Node{configCheck},
+		Children: []*Node{configCheck, configReload},
 	}
 
 	root.Children = []*Node{show, set, watch, configNode, quit, exit}
@@ -195,6 +195,10 @@ func dynHealthChecks(ctx context.Context, client grpcapi.MaglevClient) []string
 	return resp.Names
 }
 
+// dynNone marks a slot node that accepts any input but provides no
+// tab-completion candidates (e.g. a pool name or numeric weight value).
+func dynNone(_ context.Context, _ grpcapi.MaglevClient) []string { return nil }
+
 // ---- run functions ---------------------------------------------------------
 
 func runShowVersion(_ context.Context, _ grpcapi.MaglevClient, _ []string) error {
@@ -315,9 +319,14 @@ func runShowBackend(ctx context.Context, client grpcapi.MaglevClient, args []str
 	fmt.Fprintf(w, "%s\t%s\n", label("healthcheck"), info.Healthcheck)
 	for i, t := range info.Transitions {
 		ts := time.Unix(0, t.AtUnixNs)
-		lbl := ""
+		var lbl string
 		if i == 0 {
 			lbl = label("transitions")
+		} else {
+			// Pad to same visible width as "transitions" and wrap through
+			// label() so tabwriter sees the same byte count (ANSI overhead
+			// is identical on every row, keeping columns aligned).
+			lbl = label("           ")
 		}
 		fmt.Fprintf(w, "%s\t%s → %s\t%s\t%s\n",
 			lbl,
@@ -501,6 +510,26 @@ func runConfigCheck(ctx context.Context, client grpcapi.MaglevClient, _ []string
 	return fmt.Errorf("semantic error: %s", resp.SemanticError)
 }
 
+func runConfigReload(ctx context.Context, client grpcapi.MaglevClient, _ []string) error {
+	ctx, cancel := context.WithTimeout(ctx, callTimeout)
+	defer cancel()
+	resp, err := client.ReloadConfig(ctx, &grpcapi.ReloadConfigRequest{})
+	if err != nil {
+		return err
+	}
+	if resp.Ok {
+		fmt.Println("config reloaded")
+		return nil
+	}
+	if resp.ParseError != "" {
+		return fmt.Errorf("parse error: %s", resp.ParseError)
+	}
+	if resp.SemanticError != "" {
+		return fmt.Errorf("semantic error: %s", resp.SemanticError)
+	}
+	return fmt.Errorf("reload error: %s", resp.ReloadError)
+}
+
 // formatDuration formats a duration as Xd Xh Xm Xs without milliseconds.
 func formatDuration(d time.Duration) string {
 	if d < 0 {