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/tree.go

@@ -91,6 +91,36 @@ func findSlotChild(children []*Node) *Node {
 	return nil
 }
 
+// helpLine is a (path, help) pair used when displaying '?' output.
+type helpLine struct {
+	path string
+	help string
+}
+
+// expandPaths returns all (path, help) pairs for every node reachable from
+// node that has a Run function. prefix is the display string accumulated so
+// far (e.g. "show frontend"). visited prevents infinite loops through
+// self-referencing slot nodes like watchEventsOptSlot.
+func expandPaths(node *Node, prefix string, visited map[*Node]bool) []helpLine {
+	if visited[node] {
+		return nil
+	}
+	visited[node] = true
+
+	var lines []helpLine
+	if node.Run != nil {
+		lines = append(lines, helpLine{path: prefix, help: node.Help})
+	}
+	for _, child := range node.Children {
+		childPrefix := child.Word
+		if prefix != "" {
+			childPrefix = prefix + " " + child.Word
+		}
+		lines = append(lines, expandPaths(child, childPrefix, visited)...)
+	}
+	return lines
+}
+
 // Candidates returns the completable children at the current position given
 // the already-typed tokens and the partial token being completed.
 func Candidates(root *Node, tokens []string, partial string, ctx context.Context, client grpcapi.MaglevClient) []*Node {