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/shell.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"strings"
 
 	"github.com/chzyer/readline"
 
@@ -75,20 +76,28 @@ func dispatch(ctx context.Context, root *Node, client grpcapi.MaglevClient, toke
 	return node.Run(ctx, client, args)
 }
 
-// showHelp prints available completions for the given token path.
+// showHelp prints all reachable commands from the given token path, birdc-style.
 func showHelp(root *Node, tokens []string) {
 	node, _ := Walk(root, tokens)
+	prefix := strings.Join(tokens, " ")
+	lines := expandPaths(node, prefix, make(map[*Node]bool))
 
-	candidates := filterFixedChildren(node.Children, "")
-	if len(candidates) == 0 {
+	maxLen := 0
+	for _, l := range lines {
+		if len(l.path) > maxLen {
+			maxLen = len(l.path)
+		}
+	}
+
+	if len(lines) == 0 {
 		fmt.Println("  <no completions>")
 		return
 	}
-	for _, c := range candidates {
-		if c.Help != "" {
-			fmt.Printf("  %-20s  %s\n", c.Word, c.Help)
+	for _, l := range lines {
+		if l.help != "" {
+			fmt.Printf("%-*s  %s\n", maxLen+2, l.path, l.help)
 		} else {
-			fmt.Printf("  %s\n", c.Word)
+			fmt.Printf("%s\n", l.path)
 		}
 	}
 }