fix(tui): don't italicize intraword underscores in markdown

The inline markdown regex matched `_..._` / `__...__` anywhere, so file
paths like `browser_screenshot_ecc1c3feab.png` got mid-path italics.

Require non-word flanking (`(?<!\w)` / `(?!\w)`) on underscore emphasis
so snake_case identifiers and paths render literally, matching the
CommonMark intraword rule. `*` / `**` keep intraword semantics.

ui-tui/src/__tests__/markdown.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from 'vitest'
+
+import { INLINE_RE, stripInlineMarkup } from '../components/markdown.js'
+
+const matches = (text: string) => [...text.matchAll(INLINE_RE)].map(m => m[0])
+
+describe('INLINE_RE emphasis', () => {
+  it('matches word-boundary italic/bold', () => {
+    expect(matches('say _hi_ there')).toEqual(['_hi_'])
+    expect(matches('very __bold__ move')).toEqual(['__bold__'])
+    expect(matches('(_paren_) and [_bracket_]')).toEqual(['_paren_', '_bracket_'])
+  })
+
+  it('keeps intraword underscores literal', () => {
+    const path = '/home/me/.hermes/cache/screenshots/browser_screenshot_ecc1c3feab.png'
+
+    expect(matches(path)).toEqual([])
+    expect(matches('snake_case_var and MY_CONST')).toEqual([])
+    expect(matches('foo__bar__baz')).toEqual([])
+  })
+
+  it('still matches asterisk emphasis intraword', () => {
+    expect(matches('a*b*c')).toEqual(['*b*'])
+    expect(matches('a**bold**c')).toEqual(['**bold**'])
+  })
+})
+
+describe('stripInlineMarkup', () => {
+  it('strips word-boundary emphasis only', () => {
+    expect(stripInlineMarkup('say _hi_ there')).toBe('say hi there')
+    expect(stripInlineMarkup('browser_screenshot_ecc.png')).toBe('browser_screenshot_ecc.png')
+    expect(stripInlineMarkup('__bold__ and foo__bar__')).toBe('bold and foo__bar__')
+  })
+})