feat(editor): word and line movement/deletion keys

[heeseon87]

Problem

The comment editor only handles single-character Backspace/Left/Right. Every modifier combination is dropped, because the keypress handler ignores ctrl/meta and falls through to the text-insert branch guarded by !key.ctrl && !key.meta.

Terminals translate the usual macOS/Linux editing keys into readline (Emacs) sequences, which every shell and REPL already understands:

Key	Bytes sent (Ghostty defaults, iTerm2 "Natural Text Editing", Linux Alt)
Opt/Alt + ← / →	ESC b / ESC f or ESC [1;3D / ESC [1;3C
Cmd + ← / →	^A / ^E or ESC [1;9D / ESC [1;9C
Opt/Alt + Backspace	^W or ESC DEL
Cmd + Backspace	^U

So in the editor these keys either did nothing or moved one character, while they work as expected in the shell right next to it.

Change

• New src/edit-keys.ts with pure helpers (wordStart, wordEnd, lineStart, lineEnd) and resolveEditKey, which maps a readline Key to an edit action.
• Node's readline folds both xterm modifier 3 (Alt) and 9 (Super) into key.meta, so resolveEditKey inspects key.sequence to tell word moves from line moves.
• src/editor.ts applies the resolved action before the existing single-character handling; everything else is unchanged.
• test/edit-keys.test.ts covers word/line boundaries (including CJK runs and newlines) and each sequence above.

bun test (68 pass) and bunx tsc --noEmit are clean.