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/mod.rs

@@ -35,7 +35,7 @@ pub mod translate;
 
 pub use error::{DiagnosticTable, FriendlyError};
 pub use format::{catalog, Catalog};
-pub use translate::{FailureContext, Operation, TranslateContext, Verbosity};
+pub use translate::{error_hint_class, FailureContext, Operation, TranslateContext, Verbosity};
 
 // `translate::translate` and `format::translate` are different
 // callables — the former is the structured DbError → FriendlyError