refactor(ext/shared): rename REQUIRED_PILL → REQUIRED_PILL_HTML

Code-review feedback on Task 1: the _HTML suffix makes the 'this is raw
HTML, do not escape' contract obvious at every call site. Cheap to do
now (zero consumers); would be 8 diffs once Tasks 4-6 wire the constant
into the type forms.

Plan updated in lockstep so Task 4 references the new name.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

extension/src/shared/__tests__/glyphs.test.ts

@@ -15,7 +15,7 @@ describe('glyphs', () => {
     expect(glyphs.GLYPH_LOCK).toBe('⏻');
   });
 
-  it('exports REQUIRED_PILL as an HTML snippet', () => {
-    expect(glyphs.REQUIRED_PILL).toBe('<span class="req-pill">required</span>');
+  it('exports REQUIRED_PILL_HTML as an HTML snippet', () => {
+    expect(glyphs.REQUIRED_PILL_HTML).toBe('<span class="req-pill">required</span>');
   });
 });