feat: honor readline keybindings in the comment composer

[osolmaz]

Summary

The comment composer only understood arrows, Home/End, and Backspace, so shell muscle memory (Ctrl+A, Ctrl+K, Ctrl+W…) hit dead keys.
This change teaches the composer the readline/Emacs chord set that every shell, REPL, and macOS text field honors.
Line motion, word motion, word and line kills with a yank buffer, and delete-under-cursor all work now, with submit (Ctrl+S) and cancel (Ctrl+C/Esc) unchanged.

What Changed

All editing logic lives in the pure composer; the reducer only routes chords, so every behavior is unit-tested without a terminal.

• Composer ops: kill_to_end (Ctrl+K; at end of line it kills the newline like readline), kill_to_start (Ctrl+U; a no-op at column zero so lines never merge), kill_word_back (Ctrl+W, whitespace-delimited), kill_word_back_alnum (Alt+Backspace, letter/digit boundary), kill_word_forward (Alt+D), word motion (Alt+F/Alt+B, crossing lines), and a single-slot kill buffer yanked by Ctrl+Y.
• Routing: Ctrl+A/E → line bounds, Ctrl+F/B → char motion, Ctrl+P/N → row motion through wraps, Ctrl+D → delete. Kill/motion chords respect the posting freeze, and Alt chords are explicit exceptions to the guard that ignores stray Alt input; Ctrl+Alt combinations stay inert.
• README documents the chords; the text-input-keybindings skill in the tools repo is the design source.

Testing

• cargo fmt --check, check --all-targets, clippy --all-targets --all-features -- -D warnings: clean
• cargo test: 702 tests pass (~15 new: op boundaries incl. the Ctrl+W vs Alt+Backspace boundary distinction, newline-kill/yank round trips, column-zero guards, freeze interaction, Ctrl+Alt inertness)
• cargo llvm-cov --fail-under-lines 85 and executing cargo mutants --in-diff run locally; CI runs both again
• Codex review (gpt-5.6-terra, high): clean on the first pass

Risks

Low — additive bindings on an already-tested editor core; no existing chord changed meaning.

• The kill buffer is per-draft, not global across composers; consecutive kills replace rather than append (readline appends). Both are deliberate v1 simplifications.

[osolmaz]

Final report: merged as 5c96c8e. Full readline tier 1+2 in the composer with a single-slot kill buffer; boundary-heavy tests drove in-diff mutants to 0 missed (6 timeout-only = infinite-loop mutants detected by hanging; the gate now accepts cargo-mutants exit 3 for that case). Codex review clean on the first pass. Deliberate v1 simplifications noted in the body: per-draft kill buffer, replace-not-append kills.