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/walker/mod.rs

@@ -6910,6 +6910,7 @@ mod dispatch_3a_tests {
         shape: Node::Word(Word::keyword("dsltail")),
         ast_builder: dsl_builder,
         help_id: None,
+        hint_id: None,
         usage_ids: &[],
     };
     static SMOKE_SQL: CommandNode = CommandNode {
@@ -6917,6 +6918,7 @@ mod dispatch_3a_tests {
         shape: Node::Word(Word::keyword("sqltail")),
         ast_builder: sql_builder,
         help_id: None,
+        hint_id: None,
         usage_ids: &[],
     };