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/friendly/keys.rs

@@ -180,6 +180,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("help.unknown_topic", &["topic"]),
     ("help.app.quit", &[]),
     ("help.app.help", &[]),
+    ("help.app.hint", &[]),
     ("help.app.rebuild", &[]),
     ("help.app.save", &[]),
     ("help.app.new", &[]),
@@ -222,6 +223,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
         &["message", "usage"],
     ),
     ("hint.ambient_expected", &["expected"]),
+    ("hint.getting_started", &[]),
     (
         "hint.ambient_invalid_ident",
         &["kind", "found"],
@@ -299,6 +301,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.usage.rename_column", &[]),
     ("parse.usage.export", &[]),
     ("parse.usage.help", &[]),
+    ("parse.usage.hint", &[]),
     ("parse.usage.import", &[]),
     ("parse.usage.copy", &[]),
     ("parse.usage.load", &[]),