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/strings/en-US.yaml

@@ -256,6 +256,8 @@ help:
     help: |-
         help                       — show this command list
         help <command>             — detailed help for one command (e.g. `help insert`)
+    hint: |-
+        hint                       — explain the most recent error (press F1 for a hint on what you're typing)
     rebuild: |-
         rebuild                    — rebuild the project database from project.yaml + data/ (with confirmation)
     save: |-
@@ -386,6 +388,9 @@ hint:
   ambient_complete: "Submit with Enter"
   ambient_expected: "Next: {expected}"
   ambient_error_with_usage: "{message} — usage: {usage}"
+  # H2 / ADR-0053: shown by `hint` / F1 when there is nothing specific
+  # to expand on (no recent error, empty input).
+  getting_started: "Start typing a command and press F1 for a hint, or type `help` for the full command list."
   # Invalid identifier in a schema slot (ADR-0022 stage 8e
   # + the user's #5). Voice mirrors ADR-0019's "no such
   # {kind}" wording for consistency with engine errors.
@@ -617,6 +622,7 @@ parse:
     # description.
     quit: "quit"
     help: "help [<command>]"
+    hint: "hint"
     rebuild: "rebuild"
     save: "save  |  save as"
     new: "new"