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'.

proto/maglev.proto

@@ -19,6 +19,7 @@ service Maglev {
   rpc SetFrontendPoolBackendWeight(SetWeightRequest) returns (FrontendInfo);
   rpc WatchEvents(WatchRequest) returns (stream Event);
   rpc CheckConfig(CheckConfigRequest) returns (CheckConfigResponse);
+  rpc ReloadConfig(ReloadConfigRequest) returns (ReloadConfigResponse);
 }
 
 // ---- requests ---------------------------------------------------------------
@@ -53,6 +54,15 @@ message CheckConfigResponse {
   string semantic_error = 3; // set when YAML is valid but semantically incorrect
 }
 
+message ReloadConfigRequest {}
+
+message ReloadConfigResponse {
+  bool ok = 1;
+  string parse_error = 2;    // set when YAML cannot be read or parsed
+  string semantic_error = 3; // set when YAML is valid but semantically incorrect
+  string reload_error = 4;   // set when config is valid but the reload itself failed
+}
+
 message SetWeightRequest {
   string frontend = 1;
   string pool = 2;