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/complete.go

@@ -67,15 +67,16 @@ func (ql *questionListener) OnChange(line []rune, pos int, key rune) (newLine []
 		return line, pos, false
 	}
 
-	// line[:pos] includes the '?' just typed — strip it before tokenizing.
+	// Strip the '?' that was just appended to line[:pos].
 	before := string(line[:pos])
 	if len(before) > 0 && before[len(before)-1] == '?' {
 		before = before[:len(before)-1]
 	}
 	tokens := splitTokens(before)
 
-	var partial string
+	// Split into confirmed prefix tokens and the partial token being typed.
 	var prefix []string
+	var partial string
 	if len(before) == 0 || before[len(before)-1] == ' ' {
 		prefix = tokens
 		partial = ""
@@ -84,25 +85,65 @@ func (ql *questionListener) OnChange(line []rune, pos int, key rune) (newLine []
 		partial = tokens[len(tokens)-1]
 	}
 
+	// Walk the confirmed prefix to the current node, then try to advance one
+	// more step using the partial token (via prefix-match or slot fallback).
+	// This mirrors birdc: "sh?" expands "sh" to "show" and shows show's subtree.
+	node, _ := Walk(ql.root, prefix)
+	displayPrefix := strings.Join(prefix, " ")
+	if partial != "" {
+		if next := matchFixedChild(node.Children, partial); next != nil {
+			// Partial uniquely matched a fixed child — descend into it.
+			node = next
+			displayPrefix = strings.Join(tokens, " ")
+		} else if slot := findSlotChild(node.Children); slot != nil {
+			// Partial is filling a slot node.
+			node = slot
+			displayPrefix = strings.Join(tokens, " ")
+		}
+		// If partial matched nothing (ambiguous or dead end), stay at the
+		// current node and show its subcommands with the confirmed prefix.
+	}
+
+	// Expand all leaf paths reachable from the current node.
+	lines := expandPaths(node, displayPrefix, make(map[*Node]bool))
+
+	// If the cursor is at a position where the next input is a dynamic slot,
+	// fetch live values now and show them below the syntax lines.
 	ctx, cancel := context.WithTimeout(context.Background(), completeTimeout)
 	defer cancel()
+	var dynValues []string
+	var dynWord string
+	if slot := findSlotChild(node.Children); slot != nil && slot.Dynamic != nil {
+		dynValues = slot.Dynamic(ctx, ql.client)
+		dynWord = slot.Word
+	}
 
-	candidates := Candidates(ql.root, prefix, partial, ctx, ql.client)
-
-	fmt.Fprintf(ql.rl.Stderr(), "\r\n")
-	if len(candidates) == 0 {
-		fmt.Fprintf(ql.rl.Stderr(), "  <no completions>\r\n")
-	} else {
-		for _, c := range candidates {
-			if c.Help != "" {
-				fmt.Fprintf(ql.rl.Stderr(), "  %-20s  %s\r\n", c.Word, c.Help)
-			} else {
-				fmt.Fprintf(ql.rl.Stderr(), "  %s\r\n", c.Word)
-			}
+	// Right-align the help column at the width of the longest path + 2.
+	maxLen := 0
+	for _, l := range lines {
+		if len(l.path) > maxLen {
+			maxLen = len(l.path)
 		}
 	}
 
-	// Remove the '?' from the line and return with cursor one step back.
+	// Emit output. Raw terminal mode requires \r\n.
+	fmt.Fprintf(ql.rl.Stderr(), "\r\n")
+	if len(lines) == 0 {
+		fmt.Fprintf(ql.rl.Stderr(), "  <no completions>\r\n")
+	} else {
+		for _, l := range lines {
+			if l.help != "" {
+				fmt.Fprintf(ql.rl.Stderr(), "%-*s  %s\r\n", maxLen+2, l.path, l.help)
+			} else {
+				fmt.Fprintf(ql.rl.Stderr(), "%s\r\n", l.path)
+			}
+		}
+		if len(dynValues) > 0 {
+			fmt.Fprintf(ql.rl.Stderr(), "  %s:  %s\r\n", dynWord, strings.Join(dynValues, "  "))
+		}
+	}
+
+	// Remove the '?' from the line and step cursor back one position.
 	newLine = append(append([]rune{}, line[:pos-1]...), line[pos:]...)
 	return newLine, pos - 1, true
 }