Help: consume CommandNode.help_id — REGISTRY-driven in-app help

Every CommandNode declared a help_id that nothing read; the in-app
`help` body was a single hand-kept catalog block that drifted from
the command set (handoff-12 §2.1).

note_help now iterates the command REGISTRY and translates each
CommandNode's help_id (`help.<id>`), framed by help.intro /
help.dsl_section / help.types_reference. A newly-registered command
appears in `help` automatically — no edit to note_help or a hand-kept
list. Added 20 per-command help entries plus the 3 framing entries;
removed help.in_app_body.

Per-command entries use block scalars: a libyml 0.0.5 scanner bug
panics on long internal space runs in double-quoted scalars, and the
entries are space-aligned.

src/dsl/grammar/mod.rs

@@ -351,7 +351,11 @@ pub struct CommandNode {
     /// as a per-node validator (Phase A: none — every app
     /// command's ast_builder is infallible).
     pub ast_builder: fn(&MatchedPath) -> Result<Command, ValidationError>,
-    #[allow(dead_code)]
+    /// Catalog key (`help.<id>`) for this command's in-app
+    /// `help` entry. Consumed by `App::note_help`, which
+    /// iterates the REGISTRY and translates each `help_id` —
+    /// so a newly-registered command appears in `help`
+    /// automatically (ADR-0024 §help_id).
     pub help_id: Option<&'static str>,
     /// Catalog keys under `parse.usage.*` to render in the
     /// "usage:" block when a parse error fires for this command