fix(tui): inject VS16 so text-default emoji render as color glyphs

Models frequently emit bare codepoints like U+26A0 (⚠), U+2139 (ℹ),
U+2764 (❤), U+2714 (✔), U+2600 (☀), U+263A (☺) which, per Unicode, have
Emoji_Presentation=No and render as monochrome text-style glyphs in
terminals unless followed by VS16 (U+FE0F). Agent output leaked through
the TUI like `⚠ careful` instead of `⚠️ careful`.

Added `ensureEmojiPresentation` (lib/emoji.ts): scans for the curated
set of text-default codepoints and appends VS16 when the next char is
not already VS16, ZWJ, or a keycap-enclosing mark. Idempotent and
fast-pathed by a Unicode-range regex so ASCII-heavy text is untouched.

Applied once at the top of `Md`'s line parse. Hermes-ink's stringWidth
already accounts for VS16, so cursor/layout stays correct.

ui-tui/src/components/markdown.tsx

@@ -1,6 +1,7 @@
 import { Box, Link, Text } from '@hermes/ink'
 import { memo, type ReactNode, useMemo } from 'react'
 
+import { ensureEmojiPresentation } from '../lib/emoji.js'
 import { highlightLine, isHighlightable } from '../lib/syntax.js'
 import type { Theme } from '../theme.js'
 
@@ -232,7 +233,7 @@ interface MdProps {
 
 function MdImpl({ compact, t, text }: MdProps) {
   const nodes = useMemo(() => {
-    const lines = text.split('\n')
+    const lines = ensureEmojiPresentation(text).split('\n')
     const nodes: ReactNode[] = []
     let i = 0