ui: re-enable advanced-mode ambient assistance (ADR-0022 Amendment 1)

Advanced-mode hinting + completion-preview were dead: render_hint_panel
returned None for advanced mode (stale ADR-0022 §12 gate, predating the
SQL grammar) and the hint resolver/ambient_hint never threaded Mode, so a
SQL statement was gated as "this is SQL". The unified walker (ADR-0030/
0031/0032) speaks SQL, so this lifts the gate.

- ambient_hint_in_mode + hint_resolution_at_input_in_mode +
  expected_for_hint_snapshot(mode); candidate/diagnostic/parse sub-calls
  run in the active mode.
- render_hint_panel calls ambient for all modes; one-shot `:` sigil
  stripped (strip_one_shot_prefix) so `: sel` hints `select`.
- ADR-0022 Amendment 1 + README index.

Found by manual advanced-mode testing; Phase 2 marked SQL hint/completion
green at the engine layer but never exercised the UI. App-level render
test (advanced_mode_hint_panel_surfaces_sql_candidates) + ambient-layer
regression locks. 1466 baseline green.

src/snapshots/rdbms_playground__ui__tests__one_shot_advanced_dark.snap

@@ -22,7 +22,7 @@ expression: snapshot
 │                          ││: sel                                             │
 │                          │╰──────────────────────────────────────────────────╯
 │                          │╭ Hint ────────────────────────────────────────────╮
-│                          ││Type a command — press Tab for options, `help` for│
+│                          ││select                                            │
 ╰──────────────────────────╯╰──────────────────────────────────────────────────╯
 Project: Term Planner                                                           
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