refactor: remove GMAIL_CHANNEL_ENABLED env flag from add-gmail skill

Channel vs tool-only is now a code-level decision at skill apply time.
If the user chose channel mode, GmailChannel is wired unconditionally.
If tool-only, no channel code is added. No runtime flag needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/add-gmail/tests/gmail.test.ts

@@ -29,10 +29,6 @@ describe('add-gmail skill', () => {
     expect(fs.existsSync(path.join(root, 'src/channels/gmail.ts'))).toBe(true);
   });
 
-  it.skipIf(channelOnly)('config exports GMAIL_CHANNEL_ENABLED', () => {
-    expect(read('src/config.ts')).toContain('GMAIL_CHANNEL_ENABLED');
-  });
-
   it.skipIf(channelOnly)('index.ts wires up GmailChannel', () => {
     expect(read('src/index.ts')).toContain('GmailChannel');
   });