feat: H3 help <command> per-command detail + general reference

HELP node takes an optional single-word topic (BarePath);
AppCommand::Help { topic }. note_help_topic renders the help
block(s) of every command sharing that entry word (so `help
create` covers both create forms), plus `help types` and a
friendly "no help for X" pointer for unknown topics. Full help
gains a detail-hint footer. Catalogued help.detail_hint /
help.unknown_topic; parse-error matrix updated (help now takes a
topic, so the near-miss is the multi-word case). 9 integration
tests in tests/it/help_command.rs. Mark H3 [x].

src/friendly/keys.rs

@@ -174,6 +174,8 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("help.intro", &[]),
     ("help.dsl_section", &[]),
     ("help.types_reference", &[]),
+    ("help.detail_hint", &[]),
+    ("help.unknown_topic", &["topic"]),
     ("help.app.quit", &[]),
     ("help.app.help", &[]),
     ("help.app.rebuild", &[]),

src/friendly/strings/en-US.yaml

@@ -238,6 +238,10 @@ help:
   # per line so scroll math stays accurate.
   intro: "Supported commands:"
   dsl_section: "DSL data commands (in simple mode):"
+  # H3: footer on the full `help` list, and the not-found note
+  # for `help <topic>`. `{topic}` is the word the user typed.
+  detail_hint: "Type `help <command>` for detail on one command (e.g. `help insert`), or `help types` for the type reference."
+  unknown_topic: "No help for `{topic}`. Type `help` for the full command list, or `help types` for the type reference."
   # Per-command help, keyed by `CommandNode.help_id`. Block
   # scalars (`|-`) so the column alignment survives — the
   # double-quoted form trips a libyml scanner bug on long
@@ -248,6 +252,7 @@ help:
         quit                       — exit the app
     help: |-
         help                       — show this command list
+        help <command>             — detailed help for one command (e.g. `help insert`)
     rebuild: |-
         rebuild                    — rebuild the project database from project.yaml + data/ (with confirmation)
     save: |-
@@ -582,7 +587,7 @@ parse:
     # listing in `help.in_app_body` carries the user-facing
     # description.
     quit: "quit"
-    help: "help"
+    help: "help [<command>]"
     rebuild: "rebuild"
     save: "save  |  save as"
     new: "new"