feat(hint): H2 Phase A — hint command + F1 keybinding skeleton (ADR-0053)

The mechanism for the contextual hint, with tier-2 fallback; the
tier-3 corpus lands in later phases.

- new CommandNode `hint_id` field (all None for now)
- AppCommand::Hint + HINT grammar node + REGISTRY + dispatch
- F1 read-only overlay in handle_key (buffer/cursor/memo untouched)
- note_hint* renderers; hint_id_for_input_in_mode (shared selection
  helper refactored out of usage_keys_for_input_in_mode)
- last_error_hint_key + friendly::error_hint_class classifier
- catalogue: help.app.hint / parse.usage.hint / hint.getting_started
- +12 tests; 2483 pass / 1 ignored, clippy clean

src/dsl/command.rs

@@ -552,6 +552,11 @@ pub enum AppCommand {
     Help {
         topic: Option<String>,
     },
+    /// Show a contextual tier-3 hint (H2 / ADR-0053). No argument:
+    /// when submitted, it expands on the most recent runtime error
+    /// (the buffer is empty post-submit). The live-input surface is
+    /// the F1 keybinding, handled in `App::handle_key`, not here.
+    Hint,
     /// Rebuild `playground.db` from `project.yaml` + data/, with
     /// confirmation modal.
     Rebuild,
@@ -1013,6 +1018,7 @@ impl Command {
             Self::App(app) => match app {
                 AppCommand::Quit => "quit",
                 AppCommand::Help { .. } => "help",
+                AppCommand::Hint => "hint",
                 AppCommand::Rebuild => "rebuild",
                 AppCommand::Save => "save",
                 AppCommand::SaveAs => "save as",