feat(mcp): add Antigravity, fix Gemini http/sse shape, report unsupported CLIs

Formats verified against real agy/gemini/codex mcp add output.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/config/cli-registry/schema.ts

@@ -386,7 +386,7 @@ const capabilitiesSchema = z
           .max(100)
           .regex(/^[A-Za-z0-9._-]+(\/[A-Za-z0-9._-]+)*$/)
           .refine((v) => !v.split('/').includes('..'), 'must not contain ..'),
-        format: z.enum(['claude-json', 'gemini-json', 'codex-toml', 'opencode-json']),
+        format: z.enum(['claude-json', 'gemini-json', 'codex-toml', 'opencode-json', 'antigravity-json']),
       })
       .strict()
       .optional(),