feat(input): readline keymap — Esc-clear + Ctrl-A/E/W/K/U (#29)

Implement the deferred I1b readline shortcuts in the command input
field (ADR-0049, closing issue #29):

  Esc      clear a partly-typed command (only when no completion memo)
  Ctrl-A   cursor to line start (Home alias)
  Ctrl-E   cursor to line end (End alias)
  Ctrl-W   delete the previous word (readline-style, UTF-8 safe)
  Ctrl-K   kill to end of line
  Ctrl-U   kill to start of line

Esc precedence is preserved: a live Tab-completion memo still wins
(Esc undoes the completion first, ADR-0022); Esc clears only when no
memo is alive. While a sidebar panel is focused (Ctrl-O), Esc exits
navigation mode upstream and never clears the input draft. Cursor-only
keys leave history navigation intact like Home/End; buffer-mutating
keys end it like Backspace.

New helpers clear_input / delete_prev_word / kill_to_end /
kill_to_start in src/app.rs. 22 new Tier-1 tests (2458 pass / 0 fail
/ 0 skip, clippy clean). ADR-0049 amends ADR-0046's OOS list;
requirements.md I1b marked done.

docs/adr/0046-sidebar-navigation-and-responsive-input-hint.md

@@ -525,7 +525,9 @@ All tiers green, zero skips; clippy clean (nursery).
   submits over a multi-logical-line buffer. DA3/DA4 keep a single
   logical line; this remains a separate, deferred feature.
 - **Readline shortcuts (I1b)** — Ctrl-A/E/W/K/U stay reserved-deferred;
-  not touched here.
+  not touched here. *(Superseded 2026-06-12: I1b is now in scope and
+  decided by **ADR-0049** — Esc-clear + Ctrl-A/E/W/K/U in the input
+  field, issue #29.)*
 - **Cross-session sidebar persistence** — visibility is session-only
   (DB1); persisting it would amend ADR-0015.
 - **The output panel as a third navigation focus target** — navigation